I've always been told that the additives put in to raise octane flash off of the fuel really quickly. If you are buying more gas than you can use in a month it wont be any better than 87 octane from the pump. I've always bought it in smaller quantities so I always have fresh gas, even for my lawn and garden equip. and have never had any problems. I've also heard but dont know for sure that at least here in Minnesota they just add methyl alcohol to raise octane. I know we have 10% in all our fuel by law. Which has just been changed to be 20% by the end of 2008. That being said its going to be more like glow fuel that gasoline before long.