petrol is made up of a mixture of different components some lighter and some heavier. The lower fractions have more calories per litre but the higher fractions have higher octane rating. You need to blend the fractions to give the right fraction equating to the british standard for petrol at the right RON rating.
if you let some of the higher fractions evaporate then the octane level will drop, it won't vaporise in the carburettor so well or burn so evenly and the engine will run badly.
To stop standard diesel freezing in the winter people sometimes add petrol to it.
if your tank has a fair bit of diesel in it it should be fine with the remains of your petrol tin tipped in.