Found it. Since the gas goes through the same hose as the other blends, if you’re buying gas you need to get at least 4 gallons so the previous gas flushes out what was left in the hose from the previous sale, and fills enough of the container that the blend is primarily what you selected.

If you buy E15 and fill your tanks, then the next guy buys E10 and only gets 1 gallon, they might end up with a higher ethanol mix than intended, and if they use that on a small motor, like a chainsaw, it could ruin the seals.

Dont use ethanol fuel. It may damage your car, and make your car weaker.