It sounds like the starter is a bit weak or I would check the ground- wire going to the engine near the starter. If it isn't getting a good connection it will seem like the starter is going bad where as its just not getting full power.
Be sure the wires at the battery are getting a good connection also.
If the engine has been sitting for a while it might be just be seeped oil from the Rotary Valve Gear Shaft Seals. If it gets more noticable or keeps fouling plugs it could be a serious porblem. Keep us posted after you get it running and see if the oil stops after it is used a couple times.
Karl