If/when you sharpen your lawnmower blade, use one of those conical
balancer thingys and make sure the blade is level after you sharpen the
blade.Ergo a blade that isn't resonably balanced will cause vibration
and that can cause the crankshaft oil seal on the bottom to start
leaking.

A few years ago after I changed the oil in my lawnmower, I didn't have
enough of 30 weight oil to bring the oil level up to where it is suppose
to be.I added some 20 weight 40 weight oil.
That Stupid Moron old country boy from Edwards,Mississippi said, You are
putting the wrong kind of oil in there, it says use 30 weight oil only.

That MOFO Broke my chainsaw last Summer and HE tells ME what kind of oil
I should put in MY lawnmower,,, as long as I have been mowing since
around 1950!
My old lawnmower still runs as good as new.

Show me where that oil drain plug is!

http://www.devilfinder.com
Lawnmower oil drain plug
cuhulin