To be honest, they're both poor choices.

Gas because of the confusion with, er, actual gassily gaseous gas.

And petrol as a diminutive of 'petroleum', which it ain't. Not even a Morris Minor would run on a tank of crude petroleum.

My proposal as a handy term for the gasoline too many of us still put in our cars: 'Oline'.

This also has the advantage that in the event of a fuel shortage, you could go to your local refilling station and plaintively sing

Oline, oline, oline, o-line! I’m begging of you, please just fill my can!