[lunarhighway]

it also depends on the stations installation, contamination in the tanks, like water, temperatures, etc all have a small impact.

i've also heared that it's better to use stations wich are more fequently used as they always have newer gass as older gass might change slightly in constitution.

on the other hand i've also read it's not a good idea to fill up right after a station has been refuled as the new gass might have stirred up dirt particle from the bottom of the tanks that have become suspended in the fuel.

finally there have been cases over here in the past of station owners mixing cheaper lower octane fuel with higher octane fuel and selling it as the later...

i don't have any strong evidence of any of this and what the impact on your actual FE might be... but in theory all these things could create variations withing the same brand.

useing on road testing will introduce even more variables so unless the differences are really big i doubt anything other than controlled lab testing could reveal the true difference in fuels.