I am currently running 5w-20 in my 08 Civic and purchased 0w-20 this past weekend in anticipation of the next change. Both oils are Mobil 1 Extended Performance. From what I know about oils the two oils are functionally identical at operating temperatures but the 0w will be thinner at startup compared to the 5w though still much thicker than at operating temperature. Also, I think 0w-20 requires a high percentage of synthetic (Group IV) base oils so should be at least the equal if not better than the 5w-20 all other things being equal. Same Mobil EP and the same price at Walmart. It seems that the only functional difference is a thinner oil at startup. Make sense?