Wonderful! Sometimes, the best thing to do is to not do anything at all! Even though both petrol and water are liquids, petrol is lighter than water (it has a lower density). So, if you leave the mixture undisturbed, petrol and water separate on their own.
Oil and water are called immiscible liquids because they don't mix with each other. If you mix them and leave them, they get separated naturally. In other words, immiscible liquids are non-uniform (heterogeneous).
Is a mixture of milk and water immiscible?