What exactly makes one oil better than the other? I understand the differences between "Dino" and synthetic, but what seperates Amsoil 10w30 from Mobil1 10w30 from Valvoline 10w30synth? Or the difference between Rotella 15w40, Delo 15w40,Delvac 1300 15w40 and Pennzoil HD 15w40? Is it just Zinc and Phosphorous levels? ANY help would be great, I'd really like to be better educated in oils!