I worked for ExxonMobil pipeline company for a short period. It is true that all gasoline is the same. It is a commodity product. The only difference is the additive, but due to EPA standards there is not a huge difference in that either. In fact one engineer that I spoke with regarding this suggested that it would be a smart idea to change brands occasionally because of the different additives contain various detergents that work on different types of deposits. If you run the same brand all the time you stand a greater chance of building up mcertain deposits. I don't know if this is scientific or not, it was just one guys opinion. I usually run a shot of redline (non-solvent based) SI-1 cleaner occasionally in the bike and also in my cars.
If you check your local area you will find that each geographic area only distributes certain "brands" of gasoline. Then you will find that there are terminals where the trucks pick up and deliver the gas to the staions. If you watch the terminal for a few hours, you will see every "brand" of gas truck pull in and fill up. The terminals that ExxonMobil operates in my area can receive from ExxonMobil pipeline, Sun Oil pipeline or Buckeye pipeline, and likewise for the other terminals. The additive is either ExxonMobil or Generic.
I get a charge out of people who swear that their car runs better on brand X or whatever. I used to try to explain this to them, but now I figure why bother. If they think their car runs better, good for them! After all we all know how much better our cars run after an oil chnage, right? Ever heard of the placebo effect?
Of course, keep in mind that very few stations are actually owned by the oil companies. Most are franchises, and the owner may be dishonest. It is possible to "water" down gas with waste and transmix, but this will usually catch up to them in the end. I know for a fact that Mobil checks their franchisees randomly to ensure quality product. I'm sure Sunoco and any other major brand does the same to protect their reputation. It is the no-name (usually near a major highway) that get away with that sort of thing.