I also have a 2000 JGC with the 4.0 and have been using Pennzoil Platinum 10W30. I have had great results with this oil, and a Purolator PureOne filter.
I used Mobil 1 for several years when it was new, and the Pennzoil Platinum has cleaned up most of the M1 discoloration (brown) in the valve cover.
VW