German Castrol 0w30, 5000 mile OCI. 147,000 here and counting on the same truck and engine. 1999's originally used 5w30, and I like it over the 20 weights for towing. Just my opinion. I've had little trouble with the truck. Just changed the original alternator and belt this summer, and put in it's third new battery. One small problem there. The idler for the belt bolts on to the front aluminum timing cover. The steel/aluminum interface caused major corrosion, and the mounting boss broke away from the front of the plate. You may want to check yours and put a little anti seize on that stud. I have it reassembled with a washer and sleeve, but that front timing cover will need replacing for me to be satisfied. Trans has been bulletproof, and I like Wolfs Head Universal. Super smooth shifts: butt feel says smoother than Mobil 1. I suggest a spark plug change to the new style plugs. Be super careful removing and replacing, but mine came out without problems. 6 ounces of MMO in every tank of fuel keeps the fuel sender working, or will fix a sluggish or stuck sender. Mine would take forever to indicate full, but after MMO, the gauge comes up while filling. Valvoline SynPower 85w140 in the rear end works very well. Again my opinion, but I think better than RP Max Gear. Good luck with the truck. If it's half as good as mine has been, you have a winner. Mine is only used to tow the travel trailer. 9000 pounds, with a e-qual-izer hitch. Moves right on down the road, and I get 10 mpg. Hard to beat.
