I've always heard a larger filter drops your oil pressure once warmed up, also that it takes more oil to fill to capacity, becasue of larger size in which case the engine has to move more oil through the engine, which can then cause more work for the pump(which would cause worse mpg's) or heat problems(like in old vw's where if you put a deeper sump without a deeper pick up the cold oil would sink to the bottom and the hot oil would float on top of the cold and be picked up again and recirculated till the engine overheated.) There's more oil to cool, will that small amount make a difference? I don't know but those are the theories i've heard. Don't flame me
just putting in what i've heard. Also there would be more oil to warm, so wouldn't it take a little longer to heat up thus also causing lower mpg's?