Yes it is true that it is normally detergents and branding which makes the difference, but then agiain, where does all the gas come from? The same major refineries that the branded gas comes from. To maximize profits, the refineries run constantly and they push the excess to a secondary distribution channel. The el cheapo gas stations simply purchase gas on the spot market and sell as a no name brand gas. However, if your auto has specialized needs, you might wish to purchse the branded gas because there are variations between refiners in detergents and chemical levels.

Oddly enough, I used to be a public tax accountant and one of my clients was a mom and pop gas station. I was floored by how little the gas profits >10% were compared to revenue compared to the concessions sold.