5W50 is an approved weight in Mobil 1 if I'm not mistaken. From what I've read and concluded, you only need a thicker oil if oil pressure isn't sufficient at high rpms. Around 10 psi for each 1000 rpms is a good generalization. The 0 is to reduce wear at start up which is when most of the wear an engine sees in it's lifetime occurs. I've stayed with 0W40. From what I've read, it's one of the best grades Mobil makes. (Bobs the oil guy forum) I should send a sample in to Blackstone, or a similar lab, and have my used oil tested. Until tested, it's just guessing or "feels good to change". My current opinion and subject to change.

Les