I bought a 2012 Ford Fiesta for my daughter and I was surprised to see that the car calls for 5W20 motor oil. Unfortunately (or fortunately depending on how you look at it), I have several cases of QS 5W30. I guess what I don't know is if there is a real technical reason for using 5W20 or if it was just called out to meet some drubbed up CAFE standard? I know somebody's probably going to jump in here an stearnly warn me about invalidating my warranty but I really don't believe Ford would ever be able to tell the difference. Heck, my oil analysis company can't definitively tell the difference. And conventional 5W30 rapidly shears down to a 5W20 anyway. So... What it the BITOG verdict?