I found out the hard way not to buy after a hard rain at a station which is lower than its surrounding land--that is, situated at the bottom of a hill or near to one. I purchase regular for car, used to purchase regular for the Coleman stove as a test for a station. If I got blue flame, regular was good enough--yellow flame means low octane or contaminants.
During a high-price period, purchase the best you can get. I used to flea market with a fellow who once had a small fuel service, and he admitted to cutting gas with kerosene and water during high-price periods to increase his supplies and profit.
Many sites tell me that ethanol-laced gasoline will eventually eat fuel pump diaphragms. Cuz's truck just had the pump go out and he is blaming me for running the truck twice on under 1/8 tank, and letting the station go blameless for using ethanol gas.