Sheetz gas is in fact open market gas. They don’t have a consistent supplier. They buy off whoever sells it cheaper. DO they test their gasoline? Who knows? There are alot more suppliers than what you see at the gas stations. Conoco phillips ring a bell to anyone? I have heard stories that mixing gas from different suppliers will form a gel in your fuel system. I would assume that buying from sheetz exponentially increases these chances because what you pump into your car is a mixture of different suppliers fuels.