Which oil to use for my Jeep 4.0?

[jeepman3071]

Wow there is a lot of B.S. information in this thread.

1. The 4.0 DOES NOT require lots of ZDDP. Yes, it does have a flat tappet cam, but the valve springs are fairly weak. A stock engine/cam that has already been broken in will do perfectly fine on any passenger car motor oil. A 4.6L stroker motor with an aggressive cam is a different story due to the higher valve spring pressures.

2. The 4.0 oil pump IS NOT a common failure item. Many people with the 0331 head problem replace their oil pumps when they get 0 PSI oil pressure at idle, when the problem is bad bearings from coolant getting in the oil. The factory oil pumps are perfectly capable of reaching 300k miles.

3. The head cracking issue was a 1999-2002 problem on coil rail ignition 4.0 engines. XJ's and Wranglers in 1999 had distributor ignition still, so only 2000-2002 of those models had the problem. By mid 2002 the head problem was corrected.

4. The piston skirt cracking problem is not limited to '99-'02. I've seen it on a '93, '96, a few '98s, a '99 and an '02. The problem is more common on '96 and newer due to the change in piston design in 1996. The problem is that Chrysler allowed the bore dimensions in the engine to follow lax specs. Some engines are fine, others the bore is way too wide, allowing the pistons to slap back and forth. This eventually causes the pistons to crack the skirts. Some run to 300k without issue, others crack the skirt before 90k miles. The problem is hit or miss, and is even less common than the 0331 head problem. If you take care of your engine, it should be fine. Taking care of it also means avoid spending excessive time at the rev limiter.

5. As far as oil is concerned, use whatever you feel comfortable with both engine performance-wise and wallet wise. Mobil 1 is a complete waste in the 4.0L in my opinion. There are a number of UOAs that show Mobil 1 causes more iron wear in the 4.0L, so its not worth the extra coin in my opinion. I have found that both mine and another member's 4.0 engines run the quietest on Pennzoil Yellow Bottle conventional oil due to the high moly content. I have a UOA from my 4.0 with this oil and it did pretty well.

I posted a picture of my friend's Cherokee on here that had Pennzoil Yellow Bottle 10w30 its entire life, and its pretty clean inside:

Hope this helps!

 

[dlundblad]

Interesting thoughts. Since the engine is already broke in its no big deal.. Where perhaps if they were still dropping the 4.0 engines in vehicles today with the current oil formulas there "may" be more of an issue? Maybe. Maybe not. But more so than a broke in engine.

I do want to give PYB (so much cheaper!) a try, BUT..

Maybe I'm just stubborn, but id like to stay on M1 HM or TDT. Well synthetic at least.. I drive long distances in extreme hot and cold temps. Just seems like synthetic would hold up better and have better cold start flow.

I sea foamed my Jeep a few weeks back and that oil was CLEAN after I drained it. I would prefer it stay that way and just worry it'll go dirty back on conventional.

Maybe I just need schooled on conventional oils. lol.

 

[Zaedock]

Originally Posted By: jeepman3071
Wow there is a lot of B.S. information in this thread.

2. The 4.0 oil pump IS NOT a common failure item. Many people with the 0331 head problem replace their oil pumps when they get 0 PSI oil pressure at idle, when the problem is bad bearings from coolant getting in the oil. The factory oil pumps are perfectly capable of reaching 300k miles.


Some have translated my statement of "life expectency" to mean "failure". That is not what I said. The pump works, but is not optimal. The average "life expectency" of a typical 2.5L/4.0L/258 oil pump is about 150K miles. I have rebuilt MANY Jeep engines and repaired MANY ticking issues with replacement pumps. I'll stand by my statement.

 

[Hollow]

I'm taking care of a classmates 2001 GC with the 4.0.

It's due for a winter oil change, so it's getting 6 quarts of Quaker State conventional 5w30 (yay Menards sale!) and should be happy for the Chicago winter.

 

[440Magnum]

Originally Posted By: Cardenio327


I would bet the Rambler engineers that designed that engine series in 1964 never in the remotest rearmost part of the back of their head imagined a SAE 40 oil should be used in them. 1964 was the era of SAE 20 and 10w-30.


Your point is well taken (except for the SAE 20- I don't think they ever imagined that). But your history is a little wrong. The 4.0 was a CLEAN SLATE design in 1984. IT shares some dimensions (bore spacing, crank journal size and spacing, etc.) with the 258, but that was just to re-use the same assembly tooling. The fact that you can swap a 258 crank and rods into it is a by-product of those dimensions, not of the 258 itself. The 4.0 REALLY WAS a clean-sheet design, and that's according to Bill Weertman in his history of Chrysler engines. Yes, it was an AMC design, but Weertman absolutely raves about the AMC 4.0 and 2.5 that Chrysler inherited from AMC, and he was intimately familiar with their history and tooling as it fell to his department to take over production, modification, and upgrades to the design. They liked the 2.5/4.0 enough that the 2.5 displaced the Chrysler 2.5, which Weertman himself had partially designed in his earlier years before becoming head of the engine department. The 360 v8... he didn't sound so impressed. ;-) Although it should be noted that when Chrysler rolled out the 'Magnum' 5.2 and 5.9 with new cylinder heads from the previous 'LA' 318 and 360, they used the valve train almost wholesale from the AMC 360.

But back to th3 4.0 and oil: YES a 30-weight is fine for it and is what the factory always recommended. Yes, its tough and can take almost anything. But like any engine, its not so picky that it absolutely positively performs its very best with exactly a 30-weight. Furthermore a lot of experience has proven that the 4.0 does better if you skew to a heavier grade (especially a 5w40 or 0w40) than it does if you try to skew thin- to say a 5w20 or a thin 30-weight. So people that run them hard, live in hot climates (and some installations like the XJ tend to run them hot anyway) are probably wise to go a tick thicker.

 

[Clevy]

Originally Posted By: dlundblad
The only Delvac I've seen is 15w 40 at Walmart.

** Clevy I see where you're from. You skunk. You get the good stuff!



I found the gallons at napa. The 5 gallon pails are coming from an imperial oil distributor.
Now that I've seen the prices from the distributor for me to buy oil anywhere else ain't happening,unless its a crazy good sale(like almost free)
Try napa. They should have the elite 222 stuff.

 

[Clevy]

Originally Posted By: dlundblad
Interesting thoughts. Since the engine is already broke in its no big deal.. Where perhaps if they were still dropping the 4.0 engines in vehicles today with the current oil formulas there "may" be more of an issue? Maybe. Maybe not. But more so than a broke in engine.

I do want to give PYB (so much cheaper!) a try, BUT..

Maybe I'm just stubborn, but id like to stay on M1 HM or TDT. Well synthetic at least.. I drive long distances in extreme hot and cold temps. Just seems like synthetic would hold up better and have better cold start flow.

I sea foamed my Jeep a few weeks back and that oil was CLEAN after I drained it. I would prefer it stay that way and just worry it'll go dirty back on conventional.

Maybe I just need schooled on conventional oils. lol.


You are extremely misinformed. I suggest absorbing the wealth of info this site has to offer.
Unless the operating conditions are extremely severe any API oil changed at a reasonable interval will keep an engine clean. To believe otherwise is fool hardy.
The pics posted above are just more to add the the mountain of evidence proving a conventional oil like PYB is more than capable of keeping your engine clean.
Unless you plan on extending oil drains don't bother with a synthetic. Waste of money.
Believe it or not engines all over the world run in hot and cold conditions without anything special in the sump and still manage to achieve a long service life.
M1 hm is highly regarded here by those who use it. If that's what you like then have at er,but schedule drains accordingly.