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One of the questions that has plagued this site, as well as most other automotive forums, is “which engine oil should I use?” This usually turns into something resembling a religious war, mixed with “whatever’s on sale”, “canola”, and “my grandpa used this in his Model T”; the majority of the responders don’t provide any reasons other than brand trust or price point.

The last few years have seen a sudden surge in “high mileage” engine oils, targeted towards older vehicles. These days almost every major brand has a “high mileage” formulation in conventional, synthetic, and blends. Some manufacturers have even developed new oils specifically for this segment, such as Quaker State Defy. At the same time, diesel engine oils have enjoyed an upswing in popularity amongst classic car enthusiasts, as well as some vehicles that may not be really considered “classic”, but use older designs. Of all of the mainstream oils discussed, one brand comes up more often than others: Shell Rotella.



Throughout the Interwebs, Shell Rotella, specifically T6, is very well received as a high quality engine oil. In many engines it results in excellent Used Oil Analyses and displaying low wear compared to other oils. I found out a few superficial facts myself, bought into the hype and magic hand-waving of synthetic’s superiority, so tried it and was pleased with the performance.

So – what’s the difference? What makes an engine oil “High Mileage”? Why are people running diesel oil? What makes the oils different in the first place? Why are modern engine oils considered inadequate – isn’t newer supposed to better? What about backwards compatibility?

This intrigued me so I set out to find out more. I started seeing a lot about viscosity and ZDDP content. It turns out that engine oil is a little bit more complicated than it appears on the surface, and the reasons for the above are many.

I’m not going to turn this into a 9000-page physics and chemistry paper, but to understand what’s going on we need to cover a bit of background first.

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Engine oil terminology and classification

The two types you see on the shelf are commonly referred to as Heavy Duty Engine Oil and Passenger Car Motor Oil. The names alone should start to give you an idea as to the differences. They are regulated under different schemes by the API and need to meet different requirements as they are aimed at different classes of engines. HDEOs are designed primarily for diesel engines in comparatively stressful environments, whereas PCMOs are aimed at light-duty gasoline engines. For both, the standards for current oils evolve with current engines. In the automotive industry this is generally a period of about 10 years, as reinforced through various programs and social norms (ranging from road salt to Singapore).

HDEOs are classified by the API in a series of standards starting with the letter “C”, the most recent of which is CJ-4. The C stands for Compression ignition. They are formulated in viscosities appropriate for their target market, mostly 40-weight multigrades, and often meet other performance specifications for manufacturers like Mack and Cummins. These oils require excellent shear stability, wear protection, film strength, and soot control.

PCMOs are classified by the API in a series of standards starting with the letter “S”, the most recent of which is SN. The S stands for Spark Ignition. They are formulated in viscosities appropriate for their target market, mostly 20- and 30-weight multigrades, and often meet other performance specifications for manufacturers like General Motors and Toyota. These oils require good emissions performance, wear protection, and fuel economy characteristics.

The actual names of the ratings don’t matter too much, what’s important is that these oils follow slightly different sets of rules.

Over time, engine oil in general has seen tremendous improvements in performance at temperature extremes, shear strength, sludge control, anti-wear properties, emissions factors, and fuel economy. There have been new developments in refining better quality base stocks – for example Group II+ and Group III quasi-synthetics – as well as innovations in the additive package.


Let’s go over some oil basics:

Cold Cranking Dynamic Viscosity

A type of dynamic viscosity that reflects cold weather performance, measured in centipoise. Lower numbers in lower temperatures indicate an oil that flows more easily when cold, allowing the starter to turn the engine fast enough that the engine will fire.

Kinematic Viscosity

Two numbers are normally presented for an oil: kinematic viscosity in centistokes at 40°C and at 100°C, the latter generally accepted to be operating temperature. This represents the oils internal resistance to flow and how easily it will move around the engine.

High Temperature/High Shear Kinematic Viscosity

Kinematic viscosity expressed in centipoise at 150°C. This is actually a very important number as most critical areas of engines are hotter than 100°C. This reflects the ability of an oil to protect cam followers, bearings, and the cylinder walls.

SAE Viscosity Grade

The SAE has defined several grades for engine oil based on how they perform in various tests measuring their viscosity with parameters like the above. From tested viscosity, oils are sorted into various grades. Contrary to popular belief, in multigrade oils the numbers before and after the “W” actually have no relation to each other. The designations are based on testing completed.



In a nutshell, higher viscosity oils provide greater protection by way of increased film strength and shear stability, but are much more resistant to flow than lower viscosity oils. Lower viscosity oils flow better when cold and are more fuel efficient as they create less drag.

Keep in mind you do not necessarily have to forego one to get the other: When imagining a graph of how the different viscosities relate, most people picture oil viscosity as a straight line between two points. In reality, the behavior is a curve, with each formulation responding differently, effected by different base stocks and additives like viscosity index improvers and pour-point depressants.

ZDDP

Glorious zinc! Zinc is the most effective and cheapest anti-wear additive present in oils today. It has been a part of engine lubrication for a very long time, probably since the 1930s. Zinc forms a coating on top of moving metal parts to provide protection when engine oil film strength simply is not enough. This action makes it extremely useful in situations with high pressures, such as those present in a flat tappet cam follower. It also prevents corrosion due to acids for this same reason.

Similar applies to phosphorous as well, and this is important because the two metals are delivered together in compounds commonly called zinc dialkyldithiophosphates, or ZDDP. There are actually a number of different versions of these compounds and go by names like ZDP, ZDDP, ZTP, and so on.

Unfortunately, phosphorous is poisonous to the rare metals in catalytic converters, and as no internal combustion engine perfectly seals the crankcase from the combustion chamber, the oil will burn off and phosphorous will eventually kill them.

Molybdenum

Moly is an anti-wear additive that’s showing up more in engine oils. Like ZDDP, molybdenum forms a layer on metals that protects against wear and corrosion. In relation to itself it is quite slippery – not unlike graphite. Its traditional purpose is a solid lubricant in low-speed applications, such as massive bearings on large equipment, and can be found as an additive in many extreme pressure greases. Unlike ZDDP, as a solid lubricant it increases the lubricating qualities of an oil and has been shown to increase fuel economy, and is not a catalyst poison.

Boron

Boron is another anti-wear additive that’s been gaining popularity. This is primarily what companies are selling you with “ceramic” additives. Hexagonal boron nitride is often called “white graphite” and can be found in a number of other industries. Much of that which is beneficial about molybdenum applies here. There is also another additive, potassium triborate, that (for our sake) behaves in a similar way.

Calcium

Calcium is used primarily as a detergent in engine oil. It lifts deposits, soot and sludge, off of surfaces so that the oil can carry them away. Keeping the engine clean is obviously a very important function – ask anyone who dealt with sludge in the ‘70s. The detergent requirements between diesel and gasoline engines obviously vary quite a bit simply due to their difference in operation. Calcium has a couple of other effects as well: The most commonly recognized is that it increases the alkalinity of the oil. The other is that it increases the rate of decomposition of ZDDP, so too much can in fact be a bad thing.

Magnesium

Magnesium is a dispersant. Like calcium, it plays a vital role in keeping the engine clean. The two additives work very well together: Calcium breaks loose scum, and the magnesium ensures it does not settle again – like when the oil gets dumped into the sump – and is carried to the filter to be cleaned up. For contaminants smaller than the filter can handle, they can be contained in the oil longer and sludge prevented from forming in the first place. For this reason it is not uncommon for oil with a large dispersant package to appear dirtier than an oil without much dispersant: It’s because the oil is dirty, not your engine. These oils are much better suited to keeping your engine clean over a long oil change interval.

Total Base Number

The Total Base Number is an index of an oil’s reserve alkalinity. That’s right – engine oil is an alkaline liquid. The reason for this is to neutralize acids that form as a result of blow-by gasses – like sulphur and nitrous oxides – mixing with atmospheric moisture. Acids in engine oil lead to corrosion of internal parts and compromising the oil’s ability to lubricate, as well of course damaging seals. A TBN of 1-2 is generally considered the safe lower limit. As you can imagine, a high TBN is important to running an extended oil change interval.

TBN is not an additive but just a measurement. Contributing factors are many – including boron, magnesium, and calcium additives. Because of this, acid pollution is now not just a matter of prolonging corrosion, but also affecting the additive package’s ability to keep the oil clean and to protect metal. Running an oil too long will adversely affect its ability to clean and protect, and not just thin it out.

Oil Change Interval

Once upon a time, engines were very dirty and oils were crap. Older oils would shear down to a lower grade and sludge problems were common. From this arose a long-standing recommendation to change your oil every 5000 KM/3000 miles. This generic advice today is no longer accurate, and despite lube shop’s best interests, manufacturers specify different OCIs under different conditions. The truth is that most oil changed out is not yet depleted!

When examining an extended OCI there are a few factors to consider. The oil itself may actually last a good while, but additives in the oil become used up, the oil becomes acidic, and filled with particulate pollutants. Under the constant pounding and grinding of the engine, the oil inevitably shears into other molecules and is no longer within the correct viscosity range. Other pollutants appear too, such as gasoline that slips by the rings and dilutes the oil directly, and atmospheric water and coolant emulsify and displace oil, leading to wear.

The rate at which this happens varies from engine to engine. Extending your oil change intervals is a great way to save money, but requires some diligence before it should be considered.


So there’s some theory. Let’s talk about the real world.

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The Quest for Green Cars

As we idle ourselves through climate change and the price of oil shoots up there is a global movement towards more fuel efficient and lower emissions vehicles. There are of course many factors in reaching these goals, from the vehicle as a whole, the drivetrain, the design of the engine, emissions controls, etc. Of course engine oil plays a colossal factor.

The American Petroleum Institute and International Lubricants Standardization and Approval Committee have embarked on a quest to help meet this goal by improving the way engine oil performs. In the last few years several changes have been made to engine oil standards:

• Stricter limits on performance-reducing deposits and sludge
• Lower phosphorous to prolong catalytic converter life
• Fewer leaks through better compatibility with seals
• More resistance to alternative fuels like E85
• Lower oil consumption
• Fewer wear metals
• Better stability at high temperatures
• Better fuel economy

These targets have been implemented through API SM/ILSAC GF-4 in 2004 and API SN/ILSAC GF-5 in 2010. They meet their goals quite well, and, in an engine of recent design, the Resource Conserving SN/GF-5 spec performs very nicely. Synthetic oils and additives like molybdenum and boron help meet high temperature and fuel economy goals (especially with turbochargers running around), and pulling out phosphorous helps catalyst life.

There are two important things to note here:

1. To car manufacturers, who vote on these standards, the upper limit of backwards compatibility is about 10-12 years. You look at most vehicles on the road today and this really doesn’t seem like a problem. At least where I live the salt eats them alive.
2. These specifications are targeted specifically at passenger cars, and apply to 0w, 5w, and 10w variants of 20- and 30-weights. API SM- and SN-labeled oils outside of the typical PCMO range need to meet the same performance requirements, but do NOT have a phosphorous limit.

Over in HDEO land similar things are happening with regards to fuel economy, engine wear, sludge control, emissions, and so on. A big thing lately is EPA Tier 4 emissions for on-highway diesel applications, which some manufacturers are having a hard time meeting. Oil again plays a big part here as more designs use Selective Catalyst Reduction (DEF), particulate filters, and EGR setups. ULSD fuel is playing a factor here as well. Although things are changing, obviously these engines are in a different class and have different requirements than a Mazda that makes about as much torque as I do.

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The Jeep 4.0L

They say the Jeep 4.0 is like a tractor engine. This annoys the **** out of a friend of mine, but as far as the API is concerned, it might as well be.

The AMC straight-six has an illustrious history that spans back to 1958. The modern ancestor of our engine was introduced in the 1964 AMC Rambler American, available in two-door convertible trim. Hardly a tractor. In the early ‘80s AMC desired a new, more efficient engine to replace the GM Iron Duke they were buying so they drafted up the 4-banger 2.5L with a larger bore and shorter stroke. The 4.0L is actually a 2.5L with two more cylinders. As is commonplace in manufacturing, the 4.0L and 4.2L – although of somewhat different design – share many of the same parts. Engineers made what they were familiar with, so the 4.0L is essentially a design rooted in the ‘60s, redone and updated for the early ‘80s, and considered part of the AMC Straight Six family. One of the most obvious points it the flat tappet cam follower, a very traditional and cheap choice for automobiles.

What’s remarkable is that this extremely reliable engine was produced for twenty years in millions of vehicles. Starting in XJs in 1986 (’87 model year), through the demise of the TJ in 2006, the same design was manufactured the same way at the same plant. Parts listing 1987-2006 engines are no coincidence: The 1988 XJ FSM and 2005 TJ FSMs show identical internal specifications for the 4.0L. So, despite being produced as recently as 2006, this is clearly an engine from the mid ‘80s, regardless of vehicle model year.



Now, you may have noticed that this causes a bit of a problem with the API classification system as to what a “current” car is. Sure a generation of engines within a date range might give us a margin of 3 years to play with in another make, but oil certainly isn’t tested for backwards compatibility over thirty years! Even someone driving a 2006 TJ may think they are within current spec, but the actual hunk of cast iron clattering away under the hood is much more like a classic car.

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Newer Isn’t Always Better

A lot has changed since the 1980s. For example, hair spray is no longer part of a healthy breakfast, and people developed a sense of fashion. Engine oil has of course changed, too. Better sludge control, stability of the oil itself, lower emissions, etc. In general the quality of lubricants has vastly improved in the last thirty years. Of course, engine design has changed as well, and oil has evolved to match those designs. So where do our Jeeps fit in?

The biggest issue you’ll find discussed with any classic engine is the impact of low-phosphorous oils on flat tappet cam followers. As mentioned before, phosphorous gets into oil by way of the additive ZDDP. Unfortunately, lower phosphorous implies lower zinc, a key anti-wear additive. This is not good news for our engines.

Obviously for an oil to be certified under a given API or ILSAC category it must pass certain engine wear tests. These tests are performed on engines like a 1996 Buick 3800, which while also a flat-tappet push-rod design, it is not a 1983 rehash of a 1962 design. I am not an expert on engine design but this stands out to me. Ultimately, however, new oil is oriented towards new cars with roller cam followers instead, which have much lower sliding forces and pressures along the cam lobe. With reduced levels of ZDDP one would assume that molybdenum and boron additives can be added to pick up the slack for flat-tappet designs. However, this is not always the case. Some oils don’t even have them! A good coating of zinc is very important in preventing wear not only during break-in, but to maintain it throughout the life of the engine. At any rate, the SM and SN performance tests are clearly not enough for older flat tappet cam engines:





Nasty stuff.

So what can we do to avoid a similar fate in our Jeeps? More zinc is the obvious answer, but how much do we need? How much is safe? How do we get it?

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Moar Zinc

Let’s consider what our engines were actually designed for. The 1989 owner’s manual calls for “top of the line” API SG or dual-rated SG/CD fleet oil. Aside from other improvements, what has happened to zinc content since then?



As you can see, a drastic reduction in phosphorous: Obviously of great benefit to catalysts, but not so great for our engines. Phosphorous limits are defined in the standards as “non-critical”, which means they may be -10%/+15% of the written limit as long as the manufacturer “feels confident that the deviation will not harm the quality of the product”. Since lower phosphorous implies lower zinc, this means that today’s oil can have HALF of the zinc actually required by our engines when they were designed thirty years ago and during the entire production run of the XJ.

An excellent question I’ve been asked is “What about the catalytic converter? Won’t we poison it?” The answer is simple: By selecting an oil that has no greater phosphorous content than the XJ was originally sold with, we can expect at LEAST OE catalyst life. And that’s not anything to shake a stick at - we’re talking a matter of years before performance degrades. On the practical side, older vehicles are not held to the same standards through emissions testing as manufacturers have to meet with new vehicles: Even in California allowed tailpipe emissions for a 1999 are much higher than the original design target. For the rest of us, when the time comes, high-flow catalytic converters are cheap and will meet the requirements. I’d much rather install a catalytic converter every 8 years than wipe my cam in much less!

For what it’s worth, this strategy doesn’t exactly translate well to 2000+ XJ owners with the three-cat system. These mini-cats are very expensive to replace, especially in CARB land. However, there is no reason to expect any less than OE performance.

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Tractor Oil for Tractor Engines

There are a few schools of thought on what oil is best to feed to an older engine.

One of them is “always use the oil the engine was designed for”. Certainly a logical conclusion, but not all change has been bad. In the past thirty years there have been vast improvements to cold weather performance, shear stability, sludge control, detergents, and seal compatibility. If I picked up an API SG oil off the shelf today, I have no reason to expect that it won’t lower my fuel economy further, create more leaks, consume more oil, or leave my engine dirty. I want engineered performance, not 25-year-old problems! This option doesn’t make much practical sense.

Another is to grab whatever oil is on the shelf and throw in some additive to make up the difference. This is expensive and error prone, and without proper diligence you are likely shortening your catalytic converter’s life. Engine oil additives are a fine balancing act and a lot of time and money goes into formulating them to work together well. Unless you are literally a basement oil scientist you could screw things up pretty well. An alchemist might like this option.

The other is to pick up all-fleet dual-rated diesel oil. Most HDEOs are in the Xw40 range, and therefore except from parameters like phosphorous limits. This is a very good thing for our Jeeps as these oils can actually be certified to a modern standard for sludge control, seal compatibility, lower wear, and better stability; unlike so-called “high mileage” PCMOs which by simply being Xw30 are forced to certify against a lesser standard developed 18 years ago and may not meet the other requirements of a modern oil. A high-zinc oil that meets API SM or SN is basically our holy grail.

Right about now anyone who owns an XJ newer than about 1991 is something like “My owner’s manual says 5w30 or 10w30 only”. Well, let’s go back to the 4.0L’s original oil specification:

Back in the 1920s if you wanted oil you just went to Pennsylvania and put a tap in the ground. Basic oil technology has changed quite since this recommendation was done up thirty years ago, too, and is based on conventional oil that was current at the time.

The largest change in this context is shear stability. Conventional oils of old were of much lower quality base stocks, as were the additives that went in to them. Multigrade conventional oil is made by taking a lighter base stock and loading it with Viscosity Index Improvers, which artificially thicken it within a certain temperature range. This makes an SAE10 oil behave like an SAE30 when hot, yet is thin enough to achieve a rating like 10w or 5w. These poor-quality VIIs sheared easily, and do not provide the same protection at high temperature as a monograde oil would provide in the first place. The obvious solution is to specify an oil that is more tolerant to heat in warmer ambient temperatures, such as 10w30 and 10w40. This scales upwards and gives us 20w40 and 20w50 recommendation.

With modern refining techniques and newer VII technology this is no longer a problem. Even with conventional oils, there is little reason to keep changing to an oil that works worse in the cold as the ambient temperature increases. For the average owner of a vehicle in 2014, weights like 10w30 are completely obsolete and unnecessary, and it is commonplace for manufacturers to specify a single viscosity grade in all circumstances. This is reflected in the latest manuals for the 4.0L where 5w30 is the preferred choice over all temperature ranges, and 10w30 is only acceptable above 0°F.



Remember, the engines themselves haven’t changed. So we can still run 20w50 in a late-model 4.0L without any adverse effects. What has changed is fuel economy and emissions targets. There’s always a viscosity trade-off: Thinner oil is better for fuel economy but will allow more wear, thicker oil is better for protection but will soak up a little more energy.

Looking at the other hacks done to Jeep 4.0Ls it’s no surprise that every little tweak has been done to keep these things on the road. Combine this with only having to support two oil grades through the parts chain and it’s pretty easy to see why 5w30 and 10w30 are the only recommended oils in newer Jeeps… in North America, that is. Jeeps sold in export markets specify Xw40s, directly in the 2005 TJ FSM ACEA-A3 oil is listed as acceptable, where the minimum HTHS is 3.5.

So we’ve drawn a few conclusions from this information:

1. 5wXX oil is required below 0°F
2. 10wXX oil is required below 30°F
3. Up to Xw50 oil is acceptable

In other words, YES, you can put tractor oil in your Jeep!

While SAE50 multigrade oils are technically acceptable in these engines, with all of the advances in technology there’s no reason to use them outside of a racing application. Besides, we don’t need any more reasons for worse fuel economy!

I should mention that there are specific oils made for “older cars” from companies like AMSOIL. I have no doubt they are quality products, but with how much Jeep owners spend on gas and parts the idea is to stick with something affordable and on the shelves at most stores.

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Looks Like, Smells Like, Tastes Like

So far we’ve built up a pretty good picture of the sort of oil we want to put in our Jeep engines:

• Low winter rating to support cold starting
• High HTHS viscosity to protect critical parts
• Solid amount of ZDDP, not exceeding API SG, is a must
• Moly and boron anti-wear additives are a definite plus
• A strong detergent package is important

So, which oils fit the bill? Let’s ignore the religious war of conventional vs. synthetic and magic hand-waving and just look at the numbers.

Unfortunately, some numbers are not published by all manufacturers. Much of this data was collected via independent Virgin Oil Analyses from generous users on forums and companies like the Petroleum Quality Institute of America. As a result, some numbers are taken from typical production values as reported by the manufacturer, and other are taken from tests performed on a given batch. HTHS is missing for most of the conventional PCMOs because oil nerds aren’t really interested in them. Likewise, Mobil appears to not like publishing cold-cranking simulator results.

This list is by no means complete; just a few that popped into my head. If someone would like to see another oil compared drop me a line and I’ll do my best.



So much for every oil being the same! By now the real differences between HDEOs and PCMOs should be readily apparent. One of them probably tastes just like Buckley’s. Also, most of the PCMOs listed to not meet the ACEA A3 HTHS minimum of 3.5, which is specified in overseas 4.0Ls.


Conventional PCMOs

Considering modern technology, one would assume that to meet API SN-RC and ILSAC GF-5, new additives like moly and boron would be good replacements for ZDDP. Apparently some companies don’t even bother adding them!

Castrol GTX 5w30 and Valvoline Premium 5w30 I would run only in a lawn mower I hated. They have the lowest ZDDP of any lubricant in this chart, no moly or boron, and their detergent package is very low. On top of that, Castrol GTX is at the very limit for being a 5W rating, which is a good thing as I don’t mow my lawn past October. Valvoline’s product sports a very low TBN, apparently keeping alive the notion that oil changes must be done every 3000 miles.

Quaker State and Pennzoil’s 5w30s appear are what modern conventional PCMOs should look like. The viscosity data hint at different refining process or VII levels, more research is required to figure out what the real difference is. Pennzoil appears to have a superior additive package with a higher TBN, and would be a good choice in other engines.

MAG1’s 5w30, in some regions sold as Walmart SuperTech, actually appears to be a pretty good product, as far as this category goes, anyway. It has the highest level of zinc, reasonable detergent, and a chunk of moly. The TBN isn’t very good but considering the cost of this oil is not a huge deal.


Synthetic PCMOs

As far as the SN oils go, there are some unexpected differences evident between the synthetic and conventional PCMOs. ZDDP additives are tuned for more zinc with less phosphorous. As well, most of them have moly and boron to make up for lower zinc levels. This is very good news for owners, and I suspect another reason that more new cars specify synthetic oils.

Once again, Valvoline seems to be leading the way with a deficient additive package. This is remarkable as the MaxLife formula is marketed as a “high mileage” oil – yet contains no modern anti-wear additives, and none of the zinc required by older engines. It also has a low TBN so it won’t even last that long. Curiously this oil is rated API SN, though is inferior to the other SN oils in almost every way.

Quaker State’s Defy is another “high mileage” oil that appears to fall short of the mark. It too is lacking in the zinc department, but this is a real mystery on account of its API SL certification. Why is it only SL while it meets the SN limit for phosphorous? This raises some questions as to the quality of the rest of the oil. At least it has some boron and moly. Not a totally useless oil, but I’d definitely buy Pennzoil conventional before picking up a jug of QS Defy.

Mobil 1 High Mileage 10w30 is the only oil I have in this chart that actually appears to be designed for use in use in older engines. It has a pretty good chunk of zinc as well as moly and a whack of boron to back it up. Additionally it is on the high end of what is permissible for a 30-weight. The large detergent package, thicker oil, and reasonable TBN make this a good candidate for leaving it in longer in an engine from a dirtier time. This oil obviously violates the SM and SN limit for phosphorous so is certified in the SL category. One can only hope that it meets the rest of the SM and SN spec.

Castrol Edge Syntec 5w30 is a pretty solid oil that would be a good choice for an extended OCI. It has a very high HTHS for a 30-weight and the TBN shows quite the alkaline reserve. Oddly this oil has double the amount of magnesium than calcium, which will have two effects: This oil focuses on more on suspending contaminants than cleaning them, so would not do as well as other oils to clean up an already dirty engine. However, low calcium will let the ZDDP go farther before it degrades.

Pennzoil Platinum and Mobil 1 5w30s seem to take the cake in this category. Mobil 1 is probably made from better base stocks and sports superior cold cranking. Both have excellent TBNs and, as far as SN goes, good anti-wear packages. Although Pennzoil has a better ratio in the ZDDP formulation, Mobil 1 has more moly and boron to make up for it. Between these two oils we are seeing for the first time a significant difference in the cleaning package: Pennzoil is heavy on detergent, and Mobil 1 has a much more balanced approach with a whack of dispersant. Mobil 1 will be more effective in keeping contaminants suspended in the oil so that the filter can clean them up. Of these two oils, Mobil 1 appears to be better suited for running an extended OCI. For a standard OCI, the Pennzoil is likely priced much better.

Curiously Mobil 1 Extended Performance 5w30 is basically a watered-down version of Mobil 1 5w30 in a more expensive bottle.


Conventional HDEOs

Now we’re starting to get into the good stuff. The first thing I’d like to point out is that all of these diesel oils are also certified by the API for use in gasoline engines. Any oil listed here with API SM passes modern wear, performance, and seal compatibility tests, but as they are all outside of the Xw20 and Xw30 range, phosphorous limits do not apply, making them ideal candidates.

Most of these oils are 15w grades, meaning they are not suitable for winter use in our engines, although acceptable for summer and mild climates.

MAG1’s Walmart-branded SuperTech oil again impresses with superior cold cranking performance and the strongest ZDDP in this category. The high flash point and good cold-cranking results hint at a high quality base stock; I would not be surprised if this was a blend. Although the TBN is a little flaccid this is a very good oil for the price.

Mobil Delvac Super 1300 15w40 is pretty similar to the SuperTech. Although it has lower zinc, it has more moly and boron. A better detergent package and TBN mean it will go further than the SuperTech.

Rotella T Triple Protection (T3) is interesting. It has great zinc, does alright when it’s cold, and a huge slug of detergent. The 15w40 beats its 10w30 brother in anti-wear protection, both in HTHS and zinc, moly, and boron, and overall is about on par with the SuperTech for anti-wear. However, the 15w40’s phosphorous content is abnormally high – nearly a 1:1 ratio with zinc! While it does have the anti-wear properties our engines need, it compares poorly to other oils as it will shorten catalyst life with little benefit. Despite this it has a serious wad of calcium which supports its use as a cheap way to clean up a dirty engine. It is an excellent conventional HDEO but there are better options for a mixed fleet.

T3 10w30, while lacking in moly, is a reasonable oil. It’s pretty thick for a 10w30, and with a higher TBN it would be well suited for extended use. A decent zinc/phosphorous ratio is supported by lower calcium. However, the 10w30 variant isn’t as strong as other oils in terms of cleaning up and may not be the best choice for a dirty engine. I should mention that although this has higher levels of phosphorous in 10w30 grade it is still able to be certified as API SM. The reason for this is that it is primarily an HDEO, not a PCMO. When a “C” designation precedes an “S” designation, as in CJ-4/SM, the “non-critical” side kicks in. This oil would be able to achieve API SM with up to 920 PPM of phosphorous. The reverse would not be true if it was rated SM/CJ-4.

Chevron Delo 400 LE 15w40 is actually my favorite oil in this category. It’s a little thicker than the other 15w40s but has the best additive package overall. With lower phosphorous for better catalyst life while keeping zinc over 1000 PPM, this is very much in line with API SL/SJ requirements that half of XJs adhered to. A healthy dose of moly and Texas-size slug of boron should produced results similar to or maybe better than higher levels of zinc. Why buy fancy ceramic additives when you can get it in this oil for less? A balanced detergent/dispersant package rounds this oil out nicely. I will be switching to this oil once weather permits.

Overall it’s easy to see why classic car owners prefer using HDEOs. The additives are simply superior to PCMO, and in some cases the additives in conventional diesel oil out-perform synthetic gas oil – but you pay much less!


Synthetic HDEOs

Now we get into the really good stuff. As far as our engines are concerned, synthetic HDEOs are the cream of the crop for all seasons and climates. They easily meet or exceed the requirements we set out earlier for both viscosity and additive characteristics. In general synthetic HDEOs have a lighter HTHS than their conventional brethren, providing a fuel economy advantage, but with a higher quality base stock still providing superior film strength.

Shell Rotella T5 is again an “interesting” oil. The 10w30 variant is a synthetic blend, and aside from the obvious implications it appears to be identical to its conventional T3 10w30 brother.

T5 0w40 is sold up here as an Arctic Blend. It is a full synthetic. It has fantastic cold weather properties and an excellent HTHS, especially considering the 0w rating, but is a little lacking on the additive side. The calcium is quite high which will impact the practical performance of ZDDP. This oil is clearly from a previous generation as supported by the old CG-4/SJ rating. Apparently this oil is being retired and a new T6 0w40 is taking its place, however I couldn’t find any data on the T6 and T5 is still on all of the shelves. Perhaps I will revisit this in a few months.

The Mobil 1 oils, once again, are very similar. Unfortunately no CCS results have been published for these two so I cannot make any comparison or speculation as to their cold weather performance, but the reader can assume it is certainly adequate. Delvac 1 ESP 5w40 has a much higher flash point and much lower pour point so is likely a better base stock. Turbo Diesel Truck 5w40 has the weakest ZDDP of the bunch, and not much in the way of moly to show for it. Delvac 1 ESP at least has more ZDDP, in fact the zinc levels place it the third highest in our entire survey, making this a very good choice. Delvac 1 has lower calcium which will let the ZDDP perform better than TDT in a flat-tappet engine. High dispersant will keep particulates suspended for a long time. With a good filter this would be a great oil to run. TDT, with more detergent, may be better suited to an engine with sludge potential, such as frequent starts and stops from city driving. This is also suggested by TDT 5w40 marketed to consumers with diesel-powered pickups, and Delvac 1 ESP 5w40 marketed at on-highway trucks.

Valvoline Premium Blue Extreme 5w40 is essentially designed for Cummins diesels. It has a stellar level of zinc and a nice dose of moly and boron, much higher than Shell Rotella T5 and both of the Mobil oils. The slightly lower HTHS will provide ample protection but giving a fuel economy advantage over the 3.9 and 4.0 candidates. The most notable feature is the incredibly massive wad of calcium detergent. While this is very typical of high-performing HDEOs from previous generations, it will have massive impact on ZDDP in flat tappet cam engines. It’s worth pointing out at this point that the majority of heavy duty diesel engines have been using roller cam followers for a very long time – earliest I’m personally familiar with is the Detroit Diesel Series 71 from 1938 – so this isn’t a problem in the intended market. While this is undoubtedly a quality oil, I do not recommend it for use in any flat tappet engine like our 4.0L.

And so we come to the last in the lube lineup: Shell Rotella T6 5w40. This oil behaves very well in the cold, of all HDEOs compared it is second only to T5 0w40. At extreme lows it actually performs better than most 5w30 conventional oils. It’s lighter than any other 40-weight HDEO at 40°C and 100°C, giving it a fuel economy advantage, but with a strong HTHS that provides better film strength than either Mobil synthetic HDEO or Valvoline PBX. Rotella T6 has the second-highest level of zinc of any oil in this chart, but throws in a respectable wedge of moly and boron to increase lubricity and protection anyway. This oil has an extremely powerful detergent/dispersant package that will clean the engine very well and actually keep it that way. The lower amount of calcium means the ZDDP will accomplish a lot more than Rotella T5 or Premium Blue Extreme, and the zinc vs. calcium ratios are above any conventional HDEO in this lineup. Combine this superior additive package with an excellent TBN and this oil will be a solid performer for a very long time. The only down-side to this oil is the high level of phosphorous, but given the other qualities this is forgivable: In a Jeep I’d much rather the engine be as reliable as possible, so if I have to replace an emissions part a year or two earlier than with another oil, then so be it. For older XJs, this oil still falls within the original API SF-SH limits for phosphorous, and is very similar to what these engines were originally designed to use.

For the Jeep 4.0L, Shell Rotella T6 is the most ideal oil to run. It provides the best balance of engine wear protection versus fuel economy, and is the best suited to provide the most reliable performance in all situations.

The price isn’t that bad either – 5L jugs come on sale for $34 every 4-6 weeks placing it at around $6.20/L, compared to my normal price of Pennzoil 5w30 at about $4.60/L, Quaker State 5w30 at $5.40/L, Mobil 1 at $10.23/L, and Royal Purple at $11.84/L.

salad

The Proof Is In the Pudding

On paper it’s pretty clear that Shell Rotella T6 is a superior oil for use in our Jeeps. But how does it actually hold up in our engines? I was originally going to write “look it up yourself”, but I like pictures, and I like charts, so here are some pictures of charts!

Below you will find a couple Used Oil Analyses of Shell Rotella T6 used in the Jeep 4.0L, well beyond common oil change intervals, and with wear often below that of other oils. Before you try extending your own OCIs you need to examine your own situation to determine if it’s right for you. Problems you may not be aware of may limit how long you can run an oil, such as fuel or water. However most engines can go longer than what was originally specified with conventional PCMOs from twenty years ago.

salad

References and Further Reading

General

http://zddplus.labecon.com/TechBrief...Oil%20Myth.pdf
http://www.turbodieselregister.com/TDR57_Oil.pdf
http://www.synmaxperformancelubrican...n_MotorOil.pdf
http://www.hotrod.com/techarticles/e.../photo_10.html
http://www.aa1car.com/library/api_mo...ifications.htm
http://books.google.ca/books?id=uhKNFzNzRxAC
http://www.leagle.com/decision/19921...FSupp424_11161
http://papers.sae.org/2004-01-1888/
http://www.bobistheoilguy.com/forums...pics/2301129/1


Additives

http://www.penriteoil.com.au/images/...20Bulletin.pdf
http://www.opelclub.com/ZDDP2.pdf
http://schaefferoil.de/sheets/praese...actmoly_en.pdf
http://www.pqiamerica.com/calcium.htm
http://www.pqiamerica.com/TBN.htm
http://www.pqiamerica.com/zinc.htm
http://www.pqiamerica.com/magnesium.htm
http://www.redorbit.com/news/science...h_crystalline/
http://en.wikipedia.org/wiki/Zinc_dithiophosphate
http://www.bobistheoilguy.com/forums...&Number=729116
http://www.carbibles.com/additives.html
http://en.wikipedia.org/wiki/Boron_nitride
http://www.bobistheoilguy.com/forums...Number=2345516
http://www.bobistheoilguy.com/forums...tio#Post530330


Viscosity

http://zddplus.labecon.com/TechBrief...0Viscosity.pdf
http://www.hddeo.com/hthsarticle.html
http://www.machinerylubrication.com/.../oil-viscosity
http://www.bobistheoilguy.com/forums...&Number=886058
http://www.bobistheoilguy.com/forums...pics/1118409/2
http://www.bobistheoilguy.com/forums...pics/2630564/1
http://www.acccc.net/index.php?optio...7&limitstart=1
http://books.google.ca/books?id=XjTaOqEgHeEC
http://www.kewengineering.co.uk/Auto..._explained.htm


Standards

http://www.infineum.com/Documents/AP...ons%202010.pdf
http://www.opieoils.co.uk/pdfs/tech-...ifications.pdf
http://www.infineum.com/SiteCollecti...5_SN_SN-RC.pdf
http://www.api.org/certifications/en...010_120210.pdf
http://www.api.org/certifications/en..._march2010.pdf
http://zddplus.labecon.com/TechBrief...%20Engines.pdf
http://www.api.org/certification-pro...tionfinal.ashx
http://www.api.org/certification-pro...-09-01-11.ashx
http://www.api.org/certification-pro...-09-01-11.ashx
http://www.oilspecifications.org/articles/api-sn.php
http://www.bobistheoilguy.com/forums...71&type=thread
http://www.finning.ca/services/machi...Oil_Guide.aspx
http://www.swri.org/4org/d08/gastests/iiigtest/
http://www.oilspecifications.org/ilsac.php
http://www.pqiamerica.com/apiserviceclass.htm
http://www.astm.org/Standards/D4739.htm
http://www.astm.org/Standards/D5293.htm


Other Resources

http://www.jeepstrokers.com/forum/vi...t=1634&p=17114
http://www.vstrom.info/Smf/index.php?topic=1097.95;wap2
http://www.jeepforum.com/forum/f177/...-chart-679542/
http://www.pqiamerica.com/March2013P...20SUMMARY.html
http://en.wikipedia.org/wiki/AMC_straight-4_engine
http://en.wikipedia.org/wiki/AMC_straight-6_engine
http://www.pqiamerica.com/
http://zddplus.com/about-us/tech-briefs/
1988 Jeep XJ FSM
2005 Jeep TJ FSM

salad

( reserved for future use )

drhoward1988

Great Info Salad!! I remember you posting In a thread of mine about this coming.

CheapCherokee

Sounds like someone recently discovered oil forums

Radi

Thanks Salad for putting all the relevant info in one, easy to point-to place.

salad

Time for a drink