Top Tier is a marketing ploy, plan and simple. No ExxonMobil, BP, or the nation's largest refiner, Valero, but Turkey Hill Minit (sic) Mart????
As T3T4 mentions, all base mogas is the same - no matter the brand, they all get the same base from the closest refinery. Each brand's additive package is added when the tankers are filled at a terminal - a Shell terminal has Exxon additives, a Valero terminal has Costco additives, etc.
You should try to buy a name-brand gasoline from a station with high turnover for best quality, and never fill up when a tanker is delivering - water and sediment will be suspended in the fuel instead of settling above and below the gasoline.