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Misconceptions and Realities

Octane is an additive to gasoline; it is not a major portion of it
Octane makes up the majority of gasoline; between 80 and 90% in most standard automotive forms. It is a primary component of all standard gasolines. Alternative fuel mixes, containing chemicals such as Ethanol (derived from corn) still generally contain a good deal of octane if they run in standard engines.
Using a higher octane gasoline causes improved engine performance
Engines must be specially designed to gain benefits from higher octane; for other engines, it is only a waste of money.

Leaded gasoline was used because lead acted as a cooling agent for the engine valves
Lead does do this, but it doesn't do this very effectively. Its primary purpose was as a cheaper replacement for higher octane percentages. Many cars designed to run on leaded gasoline can run fine on standard, unleaded gasoline.

Chemicals with the same number of each atom and with the same generic name have the same structure and properties, whereas ones with different numbers and names have different properties
Chemicals with the same generic name, such as the two forms of Octane, are more different from each other than n-Octane is from n-Heptane, at least as far as combusting properties. Structure can be just as important as content, at least in some situations.

The burning of fossil fuels produces harmful chemicals (other than carbon dioxide, such as carbon monoxide)
Chemicals like carbon monoxide are only produced when burning is not completed. Because of the limited time in the cylinder, all of the gas cannot burn before it is released, and the unburnt gas is often in the form of these chemicals. Emissions control systems on cars try to complete the burning process.