Usually the amount added is less than the above figures. Amounts are adjusted seasonally, and to provide optimum gasoline properties for best vehicle performance. Generally oxygenates are only used where required by governmental regulations. My owner's manual warns against using gasoline containing methanol, or recommends a limit on methanol content. FormulaShell gasolines sold in the United States contain no methanol.
All gasolines have finite storage life.
If storing gasoline longer than six months, you may want to use a supplemental fuel preservative additive. If aged long enough, gasolines can form gum and varnish that can cause engine operating problems, and sometimes to the engine and the fuel tank. How long gasoline can be stored depends mostly on storage conditions. Increased temperature, exposure to air and contaminants, and the material the storage container is made of can shorten storage life. Generally, gasoline stored for longer than 6 months will likely benefit from adding a supplemental stabilizing additive, usually available at your Shell service station.