2 stroke is the most unstable.
Normal gas can oxidise if exposed to air, and different components will evaporate at different rates leaving the fuel with a higher concentration of the less volatile fractions, however, if kept in a sealed, full drum, it is fairly stable.
2 stroke contains oil. The oil reacts with the octane boosters, and reduces the octane of the fuel and the lubricity of the oil mix. If you get more evaporation of the fuel than the oil, it also ends up oil rich, enhancing lubricity, but further compromising octane. This often gives no real problem, but can result in extra wear, and can result in detonation, which can then quickly destroy an engine
Regards
pat pprimmer@acay.com.au
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