RE: AMA rule?
Sugar doesn't dissolve in gasoline. It just sits in the sump at the bottom of the tank. I suppose you could eventually clog a filter with it. Maybe if you have enough water in your gas tank you could get it to go through and burn, thereby carboning the engine up. Have you ever burned sugar? You end up with a hard chunk of black carbon.
Putting the guy in Coventry sounds reasonable and gives no legal recourse.
Jim