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!
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!