On a small, hair line crack, the engine oil pressure is going to be a lot higher than the trans line pressure to push it trough the crack. The coolant and trans fluid are a lot closer to the same pressures, so,neither would leak, or both would leak. We had a Coke delivery truck that came into the shop, and we tried to sell them an engine because it was putting oil in the radiator. They ran the thing about two years, hauling Cokes, (changing the hoses periodically) before they finally gave in to replace the engine.