I think that the only way any one brand of gas COULD give better MPG would be in the formulation....those that use a blend of ethenol get less mpg, simply because there is less energy in the fuel, due to the alcohol.
If blends are the same, and the fact is, nearly all ( or maybe ALL ) gas comes from the same refiners, then you get the same as everyone else.
Some brands add a bit of their specific additives to the bulk, mostly a sales gimmick.
Maybe your Pastor does get better MPG with the Sinclair, but if he does, I think it would be because he drives more conservatively with that brand, so the results are due to his expectations.
Mark