Ok, so I've read the FAQ's about oil and all but there is one question I still have. Right now I have 5w30 Valvoline oil in my car, I was told to run 15w40 due to the high temp summers here in AZ which can get up to 110-115 degrees. I know there is usually a drop in MPG with going with heavier weight oil but the question I have is ROUGHLY (Because I know there's no definite answer) how much of a drop do you usually see when you factor the weight of oil and outside temp when going with heavier oil?? I would imagine it's not that big?? Maybe 3-4 mpg loss??