M1 0W-40 is a fine oil but I also don't see anything wrong with staying with the factory viscosity in the High Mileage flavor. I have had good luck with Maxlife and Supertech High Mileage oils in other Jeeps (2.5/4.0L's and 134F head) and trail machines.
I'd probably be more concerned if it has a Dana30 front axle with 37's.
I'd probably be more concerned if it has a Dana30 front axle with 37's.