Seriously you guys are nuts. You must be driving f1 race cars that need special gasoline. I live in New york, all the gas is 10 percent ethanol. I have been to many different gas stations since buying my car, and guess what. None of them caused any noticeable increase or decrease in my fuel mileage.
I have used Sheetz gas when traveling in PA, and it is just like the rest of the stuff. If you stick to buying fuel at a place that is busy and moves a lot of fuel, then you won't have a problem.
It's when you buy gas at billybob's mobil with one antique gas pump in front of it, and a dirty tank underground, that is when you have a fuel related problem.
I usually buy gas when at home at Delta Sonic, the place is jammed every time I go there, and never a problem.