Yes certain standards obviously, but the better grades go to Shell, BP, Esso etc according to the tanker drivers. Have a word with taxi drivers to see where they fill up.
Try filling up your tank at a supermarket and compare the miles with a main dealer fuel. I reckon you would get about 50/80...
The supermarkets may be cheaper but the fuel is low grade. Try seeing how many miles per gallon you get from supermarket fuel compared with Shell, BP etc, it does not work out cheaper "in the long run".