I have a '99 Cherokee with the 4.0 I-6. I have used synthetic since the 5000 mile point (Mobil 1 10W30 or 5W30 until 210,000 miles with 5000 mile OCI, Amsoil 5w30 with 15,000 mile OCI from then until the current 251,000 miles). It still runs great - no engine problems ever, burns no oil at all, and still has the original oil pressure of 30 or 40 (the mechanic at the dealership noted that usually the oil pressure drops to 10 or so on the 4.0 engines he'd followed). My owner's manual recommends 10W30 in warm weather, such as yours, but the engine would probably outlast the chassis no matter what you use - if you really want to keep it a long time, look into rustproofing - the electrical charge types might be good, and require no goop be applied (not that rust is much of an issue in Jeeps, either). Changing your transmission fluid every 25,000 miles or so might be helpful, too - I've always done this and my transmission has required no attention at all, despite the 251,000 miles on it.