The standard oil used for 4-stroke engines found on petrol lawnmowers is grade SAE 30.
Synthetic variations include SAE 5W-30 and SAE 10W-30. These offer enhanced performance and higher level of protection however cost more.
If you are unsure what type of machine you have please refer to your manufacturer's instruction manual where you will find further information.
If you don't have a manual you could use these contact details for lawnmower manufacturers to obtain one.
Top Tip: You will need between 400-600ml of oil for a lawnmower engine depending on size and make. (Ride-on mower engines will normally need between 1-2 litres of oil.) Always check the manufacturer's manual for exact measurements.