Also, since petrol will float on diesel, the residual fuel in the tank is likely to be petrol-rich. The only draining method is to open the tank drain and catch the fuel mixture in a container that may need to be drained and refilled a few times.

Does gasoline float on top of diesel?

As diesel and gasoline have different specific gravities, one will float on top of the other. As I2f suggested, siphon the gasoline from the tank.

How much petrol can a diesel engine tolerate?

As a safe amount some car manufacturers will say none though the industry generally accepts 7.5% or less of petrol in diesel is safe. When you work it out for your tank capacity you will soon realise it's not a lot of the wrong fuel before it's too much.