The 4. 0 i-6 in my 93 xj seen 177,000 miles before rebuilt. The only "technical problem" was a leaky rear main, week valve springs, and the timing chain was stretched. motor popped like crazy of throttle. I had that radiator blown up twice,. ran 5000rpm racing alot. pulled the family boat for years. bolt on parts. one clutch. few dana 35's in the back. Trans lasted 176,000 before 5th gear synco broke such that if you un-loaded the transmission it would pop outa gear. I drove it like this for a bit prior to rebuild.
When I pulled it I put in the 4. 7 hesco strorker kit. the barrings, pistons, cylinder walls, cam. anything shinny. has ZERO WARE. When measured, it was in perfect factory spec!!!! after 177,000 miles! this was a family owned jeep, Was my dad's then mine, so the history was known.
Could of just got new seals, chain and springs and put it back in operation.
I used synthetic oil after I got it at 65,000 miles and always warmed up and cooled down. until the prior week of rebuilding it.
completely amazed at the condition of the parts.
BTW, I'm going to part out the jeep. the drive train is good except for the rear axle. . any takers? make me and offer. Jeep's in Detroit area. the body, not so much anymore.