Many people seem to be of the view that gasoline deteriorates with time due to evaporation of volatile components. This is not strictly correct.
These lighter components such as butane (and not ethanol as an earlier poster suggested) generate less heat in combustion than the heavier gasoline fractions. Therefore, after this evaporation, the fuel will actually have a higher octane rating than before.
In theory that makes it a better fuel. However the engine timing would have been based upon gasoline of the original octane rating, and hence missfiring can result. Not much consolation if your donkey stops over the Channel to know you've got "better" fuel
