The higher quality oils brake down slower than the cheap generic brands. If you off-road the jeep alot, get a good thick paper filter and buy your oil in bulk. Off-roading puts a horrible strain on the engine and sucks up 2-3 times as much particulate as street driving. The oil will get dirty and sludge alot faster than normal oil, and will brake down faster too. Usually when I off-road my jeep hard I change the oil every 2-3K miles, and I use some of the cheapest stuff I can find that doesn't drop oil pressure by the time I change it. On all my street cars I run synthetics just to delay the change interval. I wouldn't put synthtics into a higher milage motor that has a small leak. Synthetics have VERY good detergents that will clean out any dirt around a seal that might be helping to plug it up.

I guess the real question is.. how do you drive it?