The usual suspect is the float valve in the carb, not cutting the fuel off.
The fuel then overflows through the vent holes dribbles down the venturi into the engine, past the rings then down into the sump.
Very important not to work the engine with fuel diluted oil you can add hundreds of hours of wear in no time flat.
So clean the carb, replace the float needle and seat ( if the seat is replaceable ).
Once you have the carb sorted, get some throw away oil from Wallys, HF, Lowes or you favourite junk merchants and do a cold oil change.
Then run the engine for a few minutes and do a warm oil change including the filter.
Now go mow and change the oil for your regular oil after the first mow.
