Aspen 4 is the cleanest petrol that you can use. Aspen is made from Alkylate Petrol which is made from the gases from the top of the distillation tower which is then synthetically modified into a liquid again. The result is a petrol that is totally free from sulphur, benzene and aromatics (solvents). It is chemically inert meaning that you can leave the fuel in the machinery and your lawnmower will start next spring again. An added bonus is that it does not smell, the exhausts does not smell, it will not smoke and you are breathing a much cleaner air when you are mowing your lawn.