5w-50 if I could ever find it, this was in my old bmw, and this is what my buddy always ran in his chevy 350's.

This shit made those engines run like hell, 300rpm's lower at cruising, I got roughly 5mpg better running 5w-50 over 10w-30. And his nova was getting about 30mpg running this stuff.

And yes, thicker shit is a TON better if your vehicle leaks a lot of oil, on so many instances I've seen the leak actually stop from using thicker oil.