I have a 2014 F250 PSD, and have put 80K on her so far. Have run T6 (5w-40) for many OCIs, 15w-40 for some OCIs, and 10w-30 for only one OCI. The 10w-30 OCI was a few years ago, and it got the best mpg. I'm not too particular about mpg, so I can't recall by how much but I'd guess 0.5 mpg or so. Is it possible something else caused it - sure. Many things affect my mpg, such as cetane, temperature, fuel, oil, etc., but my belief is that the 10w-30 gets the best mpg. However, I don't use 10w-30 in general because it's not recommended for towing and bio fuel which apply to me.