99 4.0 here. I switched to castrol high mileage gtx. Driven it from 127,000-167,000 now, without these supposed leaks everyones talking about. One thing that really helped my jeep was changing to 20,000 mile rated oil filters. This cleans up the entire system slowly over time. What i do is change the oil after its darkened, add a quart, wait till it wants to fuss about 2 months usually, change oil and filter, and so on, changing oil at most 2 times a year... i dont change my filter but once a year. The oil will start running ambery for three times the distance. Instead of pitch black at 1,000 miles like typical stuff on the dip stick. And if you want. Try 4 hole injectors. When i switched, instantly noticed a reduction in the legendary knock, smoother acceleration, and better power. That translates to a longer power train life. But they will heat soak if you sit too long on a hot day ...