It looks to me like you have the same problem as I do. Petrol leaking past the needle and seat valve, filling the carb and running down the crankcase vent tube into your oil.
If you remove your oil dip stick and smell the oil, does it smell of petrol?
Its an easy fix if we have the same problem. Drop the old oil and change the filter then make sure you have a fuel shut off valve between your tank and the carb and each time you stop the mower shut the valve off for 30 seconds before you turn off the engine. That will stop fuel leaking past the needle and seat into your crank case.
Other than that clean or change your plug and let us know if it helped.