I pulled the spark plugs to replace them and the.

I pulled the spark plugs to replace them and the plug nearest the belt side of the motor is bathed in oil (the other 3 plugs and cylinders are dry). When I looked the in plug well before removing it, there was some oil on top. When I removed the plug the threads and tip were covered in oil and there is oil all down inside the compression chamber/cylinder. Is this possibly a simple valve cover gasket leak or something worse having to do with the ring and piston for that cylinder? I do see oil underneath the main crankshaft when looking down the belt side of the motor and a little oil is leaking on the ground.