you're about to get 50 replies claiming 50 different brands is best!

I like the rotella stuff too, but at 15w40, I wanted to see if something a little thinner would work for me and save a few HP too, but there's no HD diesel type oil less than a 15w40, so I picked up some royal purple from the local Jegs storefront thats 10w40. haven't run it yet though.