Blackstone Labs tackles conventional vs synthetic oils effect on mpg
 

Blackstone Labs is an oil analysis company. I know a handful of users here have used them including myself. They really do a great job at analyzing oil, how your engine is wearing, and where you might be having issues. While they have many fancy tools for analyzing oil, they don't have the best equipment for measuring mileage on cars. However, they gave it their best shot and wrote up this report on conventional vs synthetic oil for fuel economy.

Summary:
Better Mileage with Synthetic?

Good read. Thanks.

Not the best test methodology (driving several thousand miles on one type, then switching to another type), but at least they admit as much.

They did do an A-B-C-A test, which is about as good as can be done given the time and effort they were willing to expend. I wonder how much of their results are the result of ambient temperature, my own MPG changes almost exactly 1 MPG per 10 degrees F.

And my swing is much wider. In summer, the first 10 miles I drive on a flat road usually averages to about 80-95mpg, while in winter it can be into the 50's, only creeping up into the 60's and 70's after extended warmup.

In my testing of mods, I like to compare a 3-fill average to a 3-fill average from the same time period of the previous year. My fuel log shows the difference:

Switching to Mobil 1 0W-30 from Motorcraft 5W-30 gained ~ .5 MPG.

Pizza pan wheel covers made NO difference in MPG. (I don't care what others have "proven" while still "adjusting the nut behind the steering wheel"), they made NO noticeable difference in my 2007 Ford Mustang's fuel economy.

Programming the ECU in my Mustang for optimum performance with Premium (91 octane) fuel and filling it with premium fuel resulted in a loss of around 1 MPG.

Installing a 25% under-drive crankshaft pulley on the 4-Liter Cologne V-6 gave me a gain of about 1 MPG.

As always, YMMV.