Nothing but the oil grade and type recommended by GM in mine. I've used conventional Pennzoil exclusively for over 35 years now and have never had an engine out or repaired. We've put almost all of the 1.2 million miles of our vehicles on Pennzoil. My current GMC's 350 is at 262,000 and uses about the same oil between changes as it did when new. Every 2,500-3,000 they all get fresh clean oil and filter. Synthetics might be great but when something works I don't fix it.
That's why I know so little about engine rebuilds. I've never needed to.
