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Gasoline:

Raw crude oil is put through a distillation process.

The end result is a multitude of petroleum products. Gasoline is one of those. This mixture of more than one hundred different hydrocarbons is rated. The American Society for Testing and Materials (ASTM) has three categories used to group the quality of gasoline. Volatility, Octane, Additive derived. The volatility of gasoline is controlled to prevent poor engine performance. Low volatility will cause poor cold engine starting. High volatility will cause excessive vapor at high temperatures (vapor lock).

An octane number is a rating scale that indicates the gasoline's ability, to prevent
excessively rapid combustion of the last of the burning mixture in the cylinder. A
high rating would indicate a better performing gasoline. In the early years, lead
was added to control this detonation. Due to undesirable emissions, the lead 
additive was replaced in 1975. 
The quality of octane in gasoline can be measured by two laboratory procedures: 1) Research octane numbers (RON) relates to low-speed, full throttle performance, 2) Motor octane numbers (MON) relates to cruising and moderate acceleration.  
To get an octane rating for road use both methods must be added. The on- the-road equation is what appears on the gas pump. (Road On= a (RON) + b (MON) +k). Additives range from oil-soluble dyes used to color code the different grades of gasoline, oxidation inhibitors, organic rust inhibitors, water dispersers and other detergents. These additives keep the fuel system and engine running clean and smooth.