Wikipedia Answer
When I raced Motocross years ago, we found that the octane level were very low after just 2 weeks. I found this on Wikipedia, highlighted a few key points, there is much more info there.
Gasoline tends to lose oxidation and deteriorate after 4-6 months of non-use and will be for the most part useless after 1 year. While it certainly will still ignite and burn, the octane content for any engines usability is severely diminished as well the gas at this point may have also become both moisture laden with water vapor, and or particulate matter if it was not in a full and sealed tank when last used.
To recondition your newly filtered old gas add in the purchased additives as mentioned earlier and add in brand new gas in a ratio of 4-1 new to old so as to thin out the old gas with new gas. You can now reuse this gas in your 4 stroke engine car, motorcycle, lawnmower, go-cart, ATV, boat, watercraft, etc. or could use it to make 2-stroke oil mixed gas.
Please note that it is NOT recommended that you use reconditioned gas in any fuel injected engines but only in carburated engines. This is due to the octane difference variables of your re-mixed gas and the computerized sensitivity of most all modern fuel injection systems.
