My understanding of the Gas business is that the additives are added to the tankers picking up the gas. That is part of the difference in pricing. So Shell, BP, Chevron can all get gas from the same filling station but the additives are added to the tank before picking up.

ARCO has generally divested their Stations. Most are owned by independent operators. Same holds true for many stations. Margins on gas for operators of the stations are very thin. They make money when you buy a car wash, go into the store for a cup of coffee etc.

I buy gas at Costco but typically go at off hours so no lines. I also go to Vons because it is convenient and also find myself there at off hours when there are no lines.