Hey all,

I am need of some advice on changing the oil in my 98 XJ. It currently has ~227k miles, and is running 10w-30. Looking though the service manual, recommended viscosity is 10w-30, or 5w-30 in colder climates. I understand the point of lower viscosity, and it intrigues me as I do tend to travel to colder areas for skiing in the winter (I currently live in the Mid-Atlantic, and travel North occasionally). In addition I have read online that some of you are putting 5w-40 diesel oil in your 4.0L. I would love some guidance on choosing some oil to prolong the life of this engine. I currently drive it about 200-300 miles per month, so relatively low regular mileage, but it does get driven pretty much every day. I am sure this has been discussed before, but I have seen differing reports on what exactly to use on high mileage engines.

 

Oh boy, here we go. I am no expert but have done some internet reading. This is my take on it - in brief...


So context is important. Those manuals were written 25+ years ago and generally speaking, back then you would have been using conventional motor oil. The old school conventional oil used a lighter weight base stock oil and Viscosity Index Improvers to help raise the viscosity at operating temps. That's why you see the 10w30 recommended in warmer temps. It was more shear stable at higher temps than the 5w30 because the base stock oil was a thicker oil to begin with.

Fast forward to todays oils. I think many people are using synthetic oils. This is a whole 'nother ball game. And you can read til your eyes are crossed about it. I think, if you are not running an extended oil change interval and treat your vehicle with some respect, just about any premium brand 10w30 full synthetic will handle the seasonal cold temps just fine. I would make sure your getting an oil with a good additive package, zinc and all that stuff for your flat tappet cam.

You'll read about diesel oils being used because they allow for more additives than gas engine oils do. All those additive levels are regulated. Diesel engine oils have more zinc and stuff. Also, many of these 4.0 are old and high mileage so running a thicker oil can be helpful there to. Flip side is if you have a engine that is burning oil - with diesel engine oil you can ruin your catalytic converters or O2 sensors - something about phosphorus I think? But if you aren't burning oil then you should be ok with diesel oil.

I personally change oil every 3k miles in a Jeep 4.0. I know I could go longer but I don't care. My jeep - my way. So I would run Shell Rotella T6 5w40 without hesitating. I would also run Valvoline VR1 10w30, Castrol Edge 5w40, and many others because for my oil change interval and application - they will work just fine.

What have you been using for engine oil? Might just stick with it. Let it warm up before you're hard on it and you'd probably have nothing to worry about.

 

I am currently using 10w-30 synthetic, and have not had any issues with it, I went to school in Rochester, NY and my car had no problems starting even in the coldest weather we had (I think -25 wind chill, IIRC). My XJ, miraculously, has never burnt or leaked oil, so fortunately, that is not something to worry about. My goal is to make this car last as long as possible, so anything that can reduce wear and tear on the engine or transmission is something I have looked into. I have always felt like my XJ was sluggish at startup until it got above ~110*F, which I am sure is normal, but I was curious if a 5w oil would have any sort of positive effect on this and if there were any drawbacks. The thing that most concerns me is the 40 weight vs 30 weight. I was looking at the 5w-40 since in theory it should have a wider temperature range, right? Living in an area that has some relatively big temperature changes between season, having a wider range oil seems appealing to me.

 

I live in central Florida....first 200,000 miles I used 10w30 Switched to synthetic some where around 160,000 miles. Have 240,000 miles now and have been using 10w40 ...needless to say it is the high mileage versions.
I had oil slightly seeping from between the head and the block just above the distributor. After much research I found this happens to many 4.0 engines with high mileage. It was recommended to go 10w40 to see if the seeping would stop, which it has. The next choice was to have the head removed and planed.
I have never had to go inside the motor for any repairs.
I change the oil and filter every 3000 miles and have done so since owning this Jeep for 28 years.

 

10w-30 or 10w-40 are fine. These engines run hot, so its important to run a good synthetic. Amsoil is well regarded, but very expensive. This motor has flat tappet cams which have traditionally required higher concentrations of ZDDP (zinc and phosphorous) additives, so synthetic oils with high ZDDP are generally a safe bet. pH stabilizers are important if you intend to try to run motor oil for extended mileage.

As for extending engine life - keep engine temperature below 230F, and when you drive it, try to drive it long enough to fully warm up the motor so that moisture in the crankcase is evaporated from the oil.

 

Just another data point, but I run Rotella T6 5w-40 in mine with the extended size Wix filter. I forget the exact number for the filter, but it's just like the regular one but an inch or so longer for more flow and filtration capacity. Like others stated above, making sure the oil temps are up before getting on it hard (not just water temps) and not doing too many short trips where the oil never warms up are two things that go a long way to ensuring long life from your engine.

 

I run the longer WIX filter too.. 51515 I do believe its the WIX XP version specific for synthetic oils.

 

If you wish to be a fanatic, "you should buy the oil with the lowest initial viscosity you can afford"_..ie 0W synthetic (I use a 5 or 10w)

The operating viscosity is another story, I run 40w on a 250k mile engine, and change oil 7500m/12mths

fitting a bypass oil filter that handles micro size particles will prolong the life of any engine as those particles pass a full flow filter but certain sizes score the bearings

DYOR

 

I've been using Mobil 1 10w40 High Mileage. Need to be careful with high ZDDP oils, but keep in mind I ran VR1 20w50 Racing oil for the first 100K miles.

I switched over to Mobil 1 High Mileage and it started leaking... so I fixed the leaks and now it gets the good high mileage, with no leaks and runs like a top.

180K miles.

👍

 

Hey all,

Thanks for all the advice, I ended up going with 5w40 and the extended WIX filter.