Paul has no idea about fuel. Stations like Arco, Citgo, etc. use effectively a lower grade of fuel. It isn't refined as much, so the octane rating just barely meets the minimum requirements to be given a specific label (IE 91 octane). It's also not filtered as much, so there can be more debris in it. As for additives, there aren't many additives in gas. Companies like Shell, Amoco, Chevron, etc. spend more time getting rid of the poor chains of hydrocarbons to have a more pure fuel, which is why they cost more. Also, if you have a nose for fuel, smell the gas from, say, a Shell station, and compare it to the smell of gas from a 7-11. The 7-11 gas will smell like it has gone bad, while the Shell will smell more pure.