I know that running synthetic in a 6.0 is the holy grail but you dont hear much about the subject with 6.4 guys. Is synthetic the way to go on the 6.4 or is it best to just stick with 15-40. I ran T-6 Rotella in my 6.0 and liked it but I was always a bit nervous to run it in the summer towing. I realize the the 6.0 and 6.4 are a completely different animal, I want to run what is best for longevity.