Avgas Storage - Avgas instability involves multi-step reactions, some of which are oxidation reactions. Hydroperoxides and peroxides are the initial reaction products. These products remain dissolved in the fuel but may attack and shorten the life of some fuel system elastomers. Additional reactions result in the formation of soluble gums and insoluble particulates. These products may clog fuel filters and deposit on the walls of aircraft fuel systems, restricting flow in small-diameter passageways
Instability of avgas during storage is generally not a problem because of the way the fuel is manufactured, and most fuel is used within a few months of its manufacture. Storage stability can be an issue at locations where fuel is stored for occasional or emergency use. Avgas that has been properly manufactured, stored, and handled should remain stable for at least one year. Avgas subjected to longer storage or to improper storage or handling should be tested to be sure it meets all applicable specification requirements before use.
Changes that can occur during storage include:
• Air oxidation of more reactive hydrocarbons.
• Air oxidation of tetraethyl lead to form an insoluble white solid.
• Evaporation of the more volatile hydrocarbon components.
Because it is the more reactive molecules that participate in instability reactions, storage stability is influenced by fuel composition. It is also influenced by storage conditions: Instability reactions occur faster and to a greater extent at higher ambient temperatures.
Storage of avgas in high ambient temperatures presents an additional challenge. The most volatile components can evaporate from the fuel and be lost to the atmosphere. If enough of the high vapor pressure components are lost, the TEL concentration could increase to above the specification maximum, and the vapor pressure could fall below the specification minimum.
Microbial Growth - Avgas is sterile when first produced because of the high refinery processing temperatures. But it becomes contaminated with microorganisms that are always present in air and water. These include bacteria and fungi (yeasts and molds). The solids formed by microbial growth are very effective at plugging fuel filters. Some microorganisms also generate acidic byproducts that can accelerate metal corrosion.
Since most microorganisms need free water to grow, microbial growth is usually concentrated at the fuel-water interface, when one exists. Some organisms need air to grow (aerobic organisms), while others grow only in the absence of air (anaerobic organisms). In addition to food (fuel) and water, microorganisms also need certain elemental nutrients. Phosphorus is the only one whose concentration might be low enough to limit microbial growth. Higher ambient temperatures also favor microbial growth.
Microbial contamination of avgas is less common than in other petroleum products, presumably due to the toxicity of tetraethyl lead, but it does occur. The best approach to microbial contamination is prevention. And the most important preventive step is keeping the amount of water in the fuel storage tank as low as possible. No additives are approved as biocides in the major avgas specifications.
https://www.skybrary.aero/bookshelf/books/2478.pdf