Originally Posted By: CR94
Not a huge difference, but not as insignificant as the snarky posters want you to believe. The difference in viscous drag in the bearings is on the order of 10-12%. Probably on the order of 1% overall fuel consumption difference, which is difficult to measure directly in everyday driving, because uncontrolled variables cause larger differences.
Like many things on here, you'll have blanket answers as gospel and people parroting the blanket answers. The reality for me is that it can depend on the application...and especially if it's a 4 cylinder. I've owned nothing but four cylinder cars for the last 38 years and I can tell you that depending on the oil and the car, there can be a repeatable ( albeit small ) difference in MPG that's outside the realm of statistical noise.
I've had two or more cars where switching to synthetic motor and gear oil gave me an FE increase in the 3% range. ~ 1 MPG
Sometimes, it seems statistical noise is stretched to 5% on here and if inputs are then controlled to go back to losing that 5% then that's noise too with stop light, traffic, average speed, tire pressure, and intestinal gas creating the delta on both ends. My take is that you can potentially be getting 1-2 more MPG in the right circumstance but it probably wouldn't be acknowledged here.