I have three JGC's ('98, '02 & '04), and I use Pennzoil High Mileage 10w30, with a FRAM xtra-guard filter, changing oil every 6-8K, and have had no oil related issues. I've found Wal-Mart has the best prices for the oil & filter.
2004 Jeep Grand Cherokee Limited 2x4 4.0L
2002 Jeep Grand Cherokee Sport 2x4 4.0L
1998 Jeep Grand Cherokee Laredo 2x4 4.0L
"Three daughters, three Jeeps"