Originally Posted by salad

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!

Originally Posted by HyenaXJ

And so you're saying here that the Rotella T6 5w40 works in all weather? I'm planning on moving to the Northeast US this year, and I know the weather can swing quite a bit there depending on time of year.

Originally Posted by Basslicks

Reporting in for a little "testimony" for engines other than the 4.0

Changed the oil in my wife's Chrysler 300m (3.5 liter OHC v6) and it was LOOOONG overdue. Filled it with Mobile 1 Synthetic 5w30. Noticed an increase of 1.25 MPG with the first tank after the oil change.

Been using Pennzoil 10w30 in my Jeep, but after THOROUGHLY re-reading the articles at the beginning of the thread, I think I'm going to give the Rotella T6 a go. Hell, the motor already has about 250k+ on it, might as well shoot for at least 300k before the stroker build.

Originally Posted by EZEARL

Any good brand of oil of the proper weight will be fine. None will harm your engine. Now when it comes to filters it's a different story. Get a GOOD one.

Originally Posted by SirCyber

I think page one has temp recommendations for weight

Originally Posted by Bugout4x4

What's the proper weight? It gets 128 in the shade here so I run 20-50 in the summer and it needs it. Demographics could come into play here.