I would not set and run 5 gallons threw an engine on the bench, but I guess you could,, but on the other hand, as expensive as my plane and gasser engine are I would run at-least 1 gallon threw my gasoline engine on the bench,,

you can give your engine a rather nice initial brake-in on the bench for it''s first gallon, then once at the field you can tune it for more power for it''s first flight without worrying about revving it to high or to long for that first flight,,

maybe for a big 100cc or 150cc fuel gulper I would run 2 gallons,, as long as you don''t over lean an engine you have no worry about getting it hot, I mean you do have a prop on it, or at-least you should,, and yes I do agree about using a smaller prop for less of a load ..

Jim