I very seldom "fill up" my truck anymore due to high fuel costs. I posed that question to a clerk in one of the RaceTrac stations. He did not know where there gas came from other then the gasoline truck that comes and fills up the underground storages.

Pure speculation but: I'd think RaceTrack buys their gas from whomever is selling it the cheapest. Perhaps they have short or long term contracts with all sorts of conditions with certain companies. Supply and demand is everything. In short, I don't think, at any given moment, anyone can know with absolute certainity from where the gas they are about to pump has come.

We could know 'in general' where a company buys gas but I imagine this scenario: GasUp buys it's gas all the time from a company like Shell, Shell has a very busy weekend and runs low, Shell buys a million gallons of gas from Exxon to cover the gap so as to retain customers.