If there was oil in the chamber it wouldn't be wet, it would have been burnt off.
You probably have oil in the plug socket only and have pread it all over as you extracted the plug.
I don't know if the Z can leak into the socket if a minor cover gasket goes(?) but might you have accidentally poured some oil in when topping up?
IMO, whatever it is, unless your Z has been running on 5 cylinders, blowing white smoke and losing oil at a rapid rate, it won't be all that serious