Chemical composition of petrol
The composition of petrol, also called 'gasoline' or 'gas' in the USA, despite it being a liquid at room temperature, contains a complex mixture of hydrocarbon compounds selected for use in internal combustion engines.

The term "95% octane" refers to the 'octane rating', 'octane number' or 'anti-knock index' of the fuel, not the amount of 'octane' in the fuel (since 'octane' is a specific organic compound in the group of alkanes).  The octane number / rating is an 'anti-knock index', based on an arbitrary scale indexed to a liquid mixture of iso-octane (2,2,4-trimethylpentane and n-heptane). Whilst different countries have different standards for anti-knock index, in the USA, the American Society for Testing and Materials (ASTM) defines two popular indices called the 'research octane number' (RON) and motor octane number (MON). Regardless if the specific stanard, the general principle is that the higher the octane number, the more the fuel can be 'compressed' before 'auto-igniting'. Auto-ignition is not desirable in petrol engines as it causes 'knocking' of the pistons which greatly shortens their life.

What petrol (gasoline) actually contains is a blend of refinery products, including reformate (high in aromatic hydrocarbons, low in alkenes), olefinic hydrocarbons, alkylate (high in paraffinic hydrocarbons) and isomerate (high in branched isomers of pentane and hexane). Specific blends vary from manufacturer. Manufacturerers even offer a range of blends at the bowser (gas station) including 10% ethanol and 'premium' blends for high performance engines.

Refer to 'Development of a Detailed Gasoline Composition-Based Octane Model' for more information.