How do you fix lawnmower which has petrol in the oil reservoir?

For most lawn mowers, especially those with a 2-stroke engine, the petrol and gas are mixed to ensure lubrication of moving parts. 4-stroke engines (as commonly found on ride on mowers), usually have the oil and petrol separated. If you have a 4-stroke engine, then you need to disassemble it and replace the piston rings to stop the petrol from leaking into the crank case.