I've been running my 98XJ 4.0 since new, at about 5,000mi I switched to Mobil-1 10w30, I'm over 270,000mi now. We picked up two used 97TJ's, both with ~100K ballpark milage. Within a day of getting both home they got switched to Mobil-1 10W30 and Mobil-1 filters. That was over a year ago on both, no leaks. One of them, my sons, rolled and he said the engine continued to run upside down for about 3 to 5 seconds before it stalled. We rebuilt the TJ's bent body parts, installed a 5" lift and 1" body lift in april and may and he's been running it with no problems and the oil pressue is still right where it's supposed to be.
Mine was losing a quart between changes and I could never find a leak till I replaced my water pump and had the whole front of the engine open to view, found that about 4 or 6 of the bolts holding the front timing chain cover on behind the harmoic balancer had backed out and the balancer was keeping them from falling out and thats where the oil was getting out from, tightend and retorqued em down and now no more oil loss.
