Additives are found in all gasolines and are there to keep the valves and injectors clean, to keep the gasoline from creating gum in the tank, and to resist corrosion. It doesn't take much to do this since the additives in the gasoline are fractions of 1%. This may not sound like much, but it is extremely effective.
Petroleum refineries start with crude oil and make various products from there. Some of the products are gasoline components, in addition to components for diesel fuel, jet fuel, crankcase oil, grease, bunker fuel, asphalt, etc. There are typically 10 to 15 blending components that can be used in street gasoline, but all are not normally used. Usually, a final blend of gasoline will contain eight to ten components with amounts varying from 1 or 2% to 20% of the total blend of gasoline.
The components are all hydrocarbons with the exception of oxygenated components required by E.P.A. in some areas.
Racing Gasoline contain less blending components than street gasoline because on the "really good stuff" is used.
These are required by law to help improve combustion efficiency (less exhaust emissions). The two most common oxygenates are Methyl Tertiary Butyl Ether (M.T.B.E.) and Ethanol.
M.T.B.E. is an ether and is very soluble (mixes well and will not separate) in gasoline. It is normally found in gasoline at a concentration of 10 to 15%. It is a synthetic made from petroleum products. It blends like a regular gasoline component, and it will not separate in the presence of water. Unfortunately, M.T.B.E. has been banned in some states.
Ethanol is an alcohol and is found in gasoline at a concentration of 7 to 10%. It is made from grain or corn, is the choice of the environmentalists, and is not souluble in gasoline. It will separate from gasoline in the presence of water.