Agrees with supercruty: Any of the top named dino oils will work-->Delo-Rotella-Mobil Delvac.-->All 15W-40. If you live in a colder region, ya might think about pouring in a synthetic 5W-40 yr round. If ya use the dino 15W-40? Don't go past 5K miles between your Oil change. If ya use the 5W-40 synthetic? You might beable to go to within the 5K to 10K mile mark without a by-pass system installed. Make sure too use a good oil filter... like
Motorcraft FL-1995
Fleetguard LF-3630
Wix 51734
Baldwin B7039
Mobil 1 M1-601
Amsoil Ea0-99
Clean oil plays a major part in keeping your 7.3L running like new... Never skimp on a cheap oil filter.