I'm reading all the BMW specs of LL17FE vs LL01 vs LL04. New BMW engines wanting 0w12 down from 0w20 and before that 5w30/5w40s. So my question is with such a hard push to these light oils what is the real mpg gain? People say it's all because of CAFE laws but what kind of MPG difference would I see if I went from a 5w40 to say a 0w20?