I've gone to LX Forums and read quite a bit over there. I'm not planning to use Rotella. I'm going to use Mobil 1 and the factory fill for the 6.1L is a 0w40. I'm asking if it really matters as some have said it does and others not so much.
I can get 5w30 Mobil 1 cheap but will it harm anything? Even the oil cap says 0w40 is whats suggested.