petrol may vary in components somewhat dependent on local specifications and climate. generally speaking it is a mix of hydrocarbons with a boiling range of say about 30 deg C up to 160 deg C. in areas with very high temperatures the more volatile components may be left out somewhat, in very cold areas the highest boiling components may be left out and some more volatile compounds may be added to obtain cold weather startability.
to acquire the recommended knockrating most of the individual components need to meet or exceed that knockrating, which calls for predominantly branched alkane chains within the preferred boiling range. a knockrating of 95 means that the mixture has the same knockrating of a mixture of 95% 2.2.4-trimethylpentane and 5% n-octane
apart from the basefluid as described above petrol can contain small amounts of substances like detergents and dispersants to prevent fouling up of the fuel injection system or the carb used and to prevent deposits in the combustion chamber to prevent ignition by glowing deposits.
again, depending on the local specification, other components may be added in small amounts like bio-alcohol, anti-icing components, color dyes, ant-knock agents (as a substitute for the tetra-ethyl-lead as used in the past) etc. the amount and type of added components nowadays is restricted by the exhaust gas treatment system attached to the engine to prevent fouling up three way catalysts.
most times nowadays different makes of fuel are dispatched from the same depot. that does not mean all the fuels exactly the same. only the basefluid is the same and is stored in large tanks. when a tanker arrives to pick up a load, fuel is delivered from the large tanks via a blending station where the necessary proprietary "cocktail" of additives is added as per specification.
