How much impact does oil weight have on gas mileage? If I get 19mpg highway, and I'm running Mobil 1 5w30, what fuel reduction would I expect if I switch to Mobil 1 0w40? I probably won't switch, just wanted to see what others experiences are with oil weight and gas mileage. I know for instance that Ford expects between .1 and 1% increase by switching from their 5w30, to 5w20. I suspect there is less of a difference in viscosity between 5w30 and 5w20 than there is between 5w30 and 0w40 so I should expect more reduction than that range I would think. Would you guess in the 1-2% range?