[Jasen]

13-14 years ago I run 3 consecutive tanks of all major brands of gas.
85 Toyota p/u long box 2wd, 2.4l EFI 140kish miles, 12 gal fuel tank, no mods.

As I recall the results

Best to worst mpg mixed driving.

Chevron/Texico avg. 28 mpg

76/Shell 26/27 mpg

Exxon/Mobile 25/26 mpg

Arco 22/23 mpg

So in my experience, there is, at least then, a notable difference in brands.
But a .40+ cent difference off set's 1 mpg loss