Hi David, welcome to Outdoorking.
After running that mower in an over-filled condition there will be oil in the crankcase ventilation valve and through the breather hose. Some of that will have to be burned rather than draining back into the crankcase. My guess would be that may be what is happening. If it had a litre too much oil though (i.e. about three times the correct amount of oil in it) there will have been some pretty ugly solid chunks of liquid being violently hurled around in there when it was running and it is possible that damage has been done. It wasn't designed to pump solid oil like a hydraulic pump, and on the underside of the piston and rings at that.
I suggest you watch the oil level carefully - even a very small amount of over-filling, due to checking the oil on ground that isn't quite level, can result in oil being blown into the breather system every time you run it.
