That's a loaded question, it's going to depend on how the engine was maintained and if it was abused or not. The 4.0L Inline 6 is about as bullet-proof as you can get. Most of the Jeeps on this forum are well over 150k miles. My '93 has 194k on it right now and, other than using a little oil, runs great.

well that's the thing, buying used (of course) I won't really know how it was treated. But like I said before, with my dad being a mechanic, he'll probably know how to check it out well.

i got mine with 160,ooo ish o it and its got 178,000 on it still uses no oil runs like a top. love it

Good sign of a well treated vehicle? Lots and lots of paperwork! Not saying that a vehicle without a stack of reciepts wasn't well taken care of, but documented maintance/repairs tends to instill confidence. You are looking at a history of everything that went wrong with the vehicle.

But yes, trust your dad's judgement. A good mechanic can sometimes tell when something 'isn't right' just by sound/feel.