Should fuel tank(s) be topped off or empty in preparation for long-term storage?
Depending on the fuel's composition, fuel tanks should either be topped off or drained as close to empty as possible. In either case, stabilizer should be added to the fuel.
In those areas where alcohol-based fuels (e.g., ethanol) are prevalent, store your boat with as little fuel in the tanks as possible. Alcohol-based fuels break down much faster than traditional fuels and tend to promote a greater build-up of condensation inside the fuel tank. This condensation not only interferes with the proper operation of engines, it can also lead to internal corrosion of the fuel tank. Where observance of this practice is not feasible, fill the tank(s) to about 95% capacity, leaving just enough room for expansion that occurs during temperature changes. The key is to leave as little air in the tank(s) as possible, as the air will eventually become moisture-laden and further exacerbate the problem.