Toyota Prius 10% gain from switching to high mileage oil

[alex16]

I found a fellow uber driver that claims he gained 10% fuel economy from switching to high mileage synthetic oil from Firestone. I think he said he went from 42mpg to 48mpg. I told him it's more likely his driving that changed the mpg rather than the oil. He stands behind the oil changed the car, I told him I would post on here with mpg experts, I tried to explain there's way too many variables on daily driving to measure gains accurately, not sure if anyone has gained much mpg from a oil upgrade? 10%?

[ksa8907]

An increase is likely if changing from conventional to synthetic. But not 10%

[Frank Lee]

42 to 48 isn't 10% anyway. Two fails.

[Mustang Dave]

November, 2014, I changed motor oil in my Mustang. I'd been running Motorcraft synthetic blend 5W-30. I switched to Mobil 1 full synthetic 0W-30. Fuel economy gain was about 1.6% or .5 MPG.
Mobil 1 website states that switching to their synthetic 0W-** synthetic oil can give increases of 0.2% to 2.0% in fuel economy. Since I drive shorter trips in my commuting, my fuel economy increase was closer to 2%.
A 10% increase in fuel economy would require some serious mechanical and/or aerodynamic modifications and/or significant adjustments to the "nut behind the steering wheel". Synthetic oil won't make that much difference.

[Xist]

Ooh ooh! I know! He changed from 60w to 0w!

[redpoint5]

Is it perhaps getting warmer in PA, right around the time the oil was changed? Could be a dozen reasons for getting better MPG, including errors in measurement and calculation, but almost certainly not from the oil change alone.

On a related note; I got 3 MPG better on the freeway than normal while pulling a jet ski. I also traveled about 10 MPH slower and drafted a semi, with my wife tailgating since my trailer lights weren't working.

[mcrews]

didn't happen.
placebo effect. He 'wanted it' so he got it.