Ethanol is a bio-fuel, whilst Methanol is a poisonous wood alcohol.
Both raise octane ratings, but are by no means standard additives. Gasoline does loose its properties over time, because some components evaporate easily, especially those that enable easy starting. If these lighter fractions were used to increase octane ratings, then the residual fuel may indeed not meet our requirements any more.
Thus, it is wise to keep only small quantities, and mix with oil as you need it.
Keeping in par with spec's is one of the main advantages of Aspen fuel (an Alkylate gasoline without the nasty smell) It has only one fraction type, and will not go stale. It also will maintain its properties in a mix, if kept over time.