If your lawnmower is leaking oil, it may be due to a broken or damaged oil seal. The oil seal is rather easy to replace; however, you must be sure to have a replacement one available that fits your exact model. You can purchase a new oil seal from most major hardware stores, or order it directly from the manufacturer.
How to Replace a Lawnmower Oil Seal
Turn the lawnmower over, exposing the underside.
Remove the blade and blade adaptor by unscrewing them. Set them aside to be replaced shortly.
Locate the broken or damaged seal underneath, and remove it. Use the flathead screwdriver to pry the seal off; cut it off if you need to.
Put your new oil seal in place, and make sure it is tightly secured. Use the edge of the flathead screwdriver to push the entire seal down tightly around the edge.
Replace the blade adaptor and blade, making sure the screws are tightly in place.
Turn the lawnmower back over, and power it on.