Someone wrote 2 weeks ago to the Wall Street Journal car column about their motorcycle needing premium gas and they were concerned the last person at the pump used regular. The answer was that about 1/3 of a gallon of the last person's choice goes in your tank. So if you buy premium, you might be getting 1/3 gallon of regular. If the motorcycle only buys 1 gallon, this is significant, according to the column.
So sit there and wait for someone to get premium then get yours