I try to buy 100% gas for my jon boat and mowers. There is a small conv. store not too far away that sells only 100%. My boat gas tank is a 3 gal tank running on a 10hp merc 4 stroke. I run 1/2oz of marine sta-bil to a tank and haven't had any problems. I usually pour whats left in the tank into my riding mower after each outing and it gets used up quickly. Over the winter, I run the boat engine dry before storage and pour the remaining gas into my truck. I never keep gas very long.
I do take my motor in every year to the dealer for a check over and fluid changes and they make sure everything is running perfect.