A reliable source told me one time, and correct me if I'm wrong....
Premium gasolines have higher levels of octane, lower grade gasolines have less... this we know, but gas sold at discount stations, for less price, than say, Shell or Mobil, usually are fuels that are transported in "Multi Use Tanks".
This is where the same tank is used for Kerosene, Diesel, and other petrol products. The reason it's discounted is that trace amounts of other fuels can be found in the gas, therefore decreasing it's purity. (And sale price)
In general, the fuel quality has decreased in the past few decades, and with the introduction of Ethanol, it's even worse.
My two cents.
