Having been in the transportation biz for some time, I can assure you that it all comes outta the same horse. Octane is really all that matters.
Tankers frequently are rerouted to different stops, and a load that was destined for Mobil and had their additives added will wind up at Hess, or 7-11 or Shell. The days of these companies being able to isolate their fuel is long past.
Regarding any Gas Retailer using only domestic product, I would be hard pressed to believe it. They swap inventory on paper on every city they serve.
They dont care where it came from as long as the wholesale $ is right.
45Ag106.......shaka! 8)