Just a thought! A friend of mine had a situation the same as yours. He took it to the shop and they found that he had left the mower outside in the rain and water had gotten into the crank case. That winter the water froze up and caused a swell and crack in the thin area of the crankcase. He noticed the water in the oil that spring and changed the oil. He ran the mower a month before he noticed the crack and took it to the shop. It does not leak oil and still runs fine. Maybe it is a cold weather crack?