An octane number is a measure of gasoline's ability to resist pre-ignition, also known as "knocking" or "pinging".
Octane is a rating, similar to a ratio of measurement, not a substance chemical or product you can add to gas.
Determining octane measurements involves a complex methodology.
Gas is available at different octane measurements - E.G. 87, 89, 91 or 93.
Portable octane analyzers (testing equipment) is available, but their use by the public is limited due to a cost range of 2-10 thousand dollars.
For more information on octane analyzers visit Shatox or Zeltex comppany websites.
"Gas Purifying Equipment" also is available, but like octane analyzers their high price tags often limit use by the general public.
While it's true that pure ethanol has a higher octane number, (reason ethanol is ingredient used most often in octane enhancer products),
E10 gas does not have an octane higher than conventional gasoline.
(Chose grade of fuel at pump for octane you prefer).
When fuel absorbs moisture the octane reading can dramatically decrease.
Fuel that has experienced water contamination (W/C) and phase separation (P/S) will decrease about 3 octane points...
Many refer to this as "lean fuel" or "diluted fuel".
If you purchased fuel with an 87 octane level, after W/C and P/S will decrease to only 84, (an unsafe level for most engines).
Proper management when using alcohol-blend fuels will avoid this problem.
Some people chose to purchase a higher level octane blend when purchasing E10 fuel - to prevent octane from reaching a dangerously low level if fuel contaminates.
To avoid wasting money on gas that will contaminate quickly always purchase high quality fuel, follow necessary precautions, and refill tank often.
Replacing fuel in your tank every 1-2 weeks will usually limit the occurence of PS and WC.
Of course, there are other situations when irregardless of how fresh fuel is, the fuel will go bad - causing decrease in octane and other negative effects on ingredients contained in fuel.
Running on gas with the appropriate octane rating will offer best performance and increase the useful life of the engine.
To maintain the octane rating listed at time of purchase, following simple ethanol precautions to protect fuel quality.
Precautions listed on this page, http://www.fuel-testers.com/ethanol_engine_precautions.html
should be all you need to do to assure engine is running on octane level recommended by engine manufacturer.