I had an 89 xj with 225k, and currently have a 98 zj with 136k. I have always used Valvoline 10w-40, or CarQuest 10w-40 (generic Valvoline). Last time I checked the xj was still going strong.
2003 Dodge CTD Quad Cab 4x4, 320k miles and counting
98 ZJ Limited, 5.2L, NP242, Jet Stage II (worthless), K&N, 245/75-16 BFG KO's
89 XJ, stock
79 CJ7 - Options included ripped soft top, leaky 4.2L (5qts/week), ripped seats, 33" tires, bondo & gray primer