Well I try to follow the manual for the machine. For instance the Honda mower says "This engine is certified to operate on unleaded gasoline with a pump octane rating of 86 or higher. Unleaded gasoline produces fewer engine and spark plug deposits and extends exhaust system life." But sometimes when I'm filling up the truck and the gas can for the other machines I just use the same pump for convenience. More important is to change oil, plug and filter frequently. Unlike the computer driven Cayman I have to adjust my mowers and other machines for high alitude to get the best out of them.