First off I would like to say what a wealth of information this site is! I've been digging changing my own oil since I was 14. It's good to know that there are others out there with my addiction. I have a 97 GMC Z71 with the 350 engine. I've always run syn. oil since I got it with 31000 miles on it. Now it has 80000 and I was thinking about going to a heavier oil ( 15w50 mobil 1 or 15w-40 delvac 1300). I've been going maybe 5500 to 6000 miles on each change. I was worried about the shearing of the mobil 1 but at the same time had thoughts about the non-syn delvac holding up. Any ideas from the "Lords of Lube" out there? o yeah, I use puroilator filters by the way.