Let me share some of my experience with 5w-20 and 5w-30 oils in two autos, one v6 and one a 4 cylinder.
The v6 "required" 5w-20 motorcraft in particular. We ran that for the first 10k miles or so. After which we tried GC, M1 10w-30 and Havoline non synthetic. MPG increased with each over the...