Originally Posted By: Branson304
I am firm in the belief that any fuel economy differences you get between different oils is going to be unnoticeable and irrelevant. Even switching viscosities isn't likely to give a noticeable fuel economy difference. I have switched to 5W-30 in 5W-20-spec'd engines & have noticed no difference in fuel economy.
You will see a difference in short trip mileage when stepping up to heavier oils, but it is highly unlikely that there will be any measurable difference on longer trips. Likewise you probably won't see any difference in long trip performance when switching to a lighter oil.
Some cases in point: our 2010 Civic was switched from Mobil1 5w20 to Mobil1 0w20 EP a little over a year ago. We've seen no change at all in MPG, no matter what the conditions. Our 2001 Volvo was changed from Mobil1 5w30 High Mileage to Mobil1 0w40 at the beginning of this summer for towing season. Short trip mileage is down 2 MPG, or close to 10%. Regular commuting, or 19 miles each way, the mileage is unchanged. On one lengthy highway trip the mileage was actually better than usual, but that is a single data point.