Guys.... the web is free to search for answers... as is AS..
The stability of AVGAS has nothing to do with the lack of alcohol... It's due to the compounds selected and blended into the gas. In a post a year or two back on AS Bwalker explained the process.
Here's a snip I found on the web:
Aviation Gasoline Notes
Content
Aviation gasoline, avgas, is required to be an all hydrocarbon product. That is, its components must be chemicals that contain only carbon and hydrogen atoms. The use of oxygenated, chemicals that include oxygen atoms, compounds such as alcohols or ethers, is not permitted. Only a few select additives are permitted and their use is strictly controlled and limited. The primary ingredient in avgas is isooctane. This is a special component produced in the refining process by specialized equipment. Small amounts of isopentane and aromatic (ring) compounds are also used. The isopentane allows the correct volatility to be achieved in the final fuel blend. Aromatics are used to improve the rich mixture ratings. However, these aromatics must be limited to achieve other specifications. Grade 80 avgas may also contain straight-run gasoline but this component's lower octane rating makes it unsuitable for higher octane blends. Approved additives include alkyl-lead anti-knock additives. Other additives are also used to then control lead deposit formation. Color dyes are required in most grades for safety identification. Another common, and required, additive includes oxidation inhibitors to improve storage stability and inhibit gum formation. Theses anti-oxidant additives also help prevent lead compound precipitation (separation). Other additives such as corrosion inhibitors, fuel system icing inhibitor and static dissipator additives may also be included by agreement with the user, by the military or by some foreign specifications. All other additives are forbidden.
Physical and Content Specifications
The density and heat of combustion of aviation gasoline is specified. However, other specification, such as distillation range, greatly limit any variability in these measurements. The freezing point of the gasoline is specified to prevent the formation of solid hydrocarbons during prolonged cold soak at altitude. The formation of solids would of course jeopardize fuel flow and prevent full fuel availability. Avgas must also be stable in storage and under a variety of conditions. A primary form of deterioration is the formation of "gum" through oxidation and polymerization of fuel molecules. These can deposit on fuel system components and cause serious problems. Therefore gum formation is strictly limited in the specifications. The addition of anti-oxidants is required and results in good to excellent storage stability. The sulfur content of avgas is limited to a very small amount. This is needed because sulfur can cause a deterioration in the anti-knock performance of the lead additive. Sulfur also contributes to corrosion of fuel system and engine components. Fuel corrosiveness is tested by a cooper strip corrosion test. Avgas is also tested for water reaction. In this test samples of fuel and water are mixed and resulting changes are noted. This is used to detect the presence of high octane but water soluble components such as alcohols in the fuel. These are not permitted and can be detected by both volume change and phase separation in the test sample
And there is much more to read -
http://purvisbros.com/afi/
