Which property are crude oil fractions separated using?

Fractional distillation is a technique that separates hydrocarbons in a mixture (crude oil is a mixture of hydrocarbons) based on their boiling points. The boiling point of a hydrocarbon depends on the length of the hydrocarbon chain, i.e. how many C atoms there are in a chain, (and whether it is branched or not..but that falls into A level Chemistry). So hydrocarbons with similar boiling points and chain length will end up in the same fraction.