What about the possibility of a crack in head gasket between oil passages and coolant passages. Oil pressure would be higher than cooling system pressure, thus oil would be more likely to enter cooling system, than coolant enter oil /lube system. Just a thought. Have you done a cooling system pressure test, just to see if any drop at all in pressure? Seeing how engine would not be running, oil pressure would be 0, and may allow coolant to seep through same path, only from cooling system, to oil. Is it a large amount that your getting?

Check your intake manifold and gasket. this will cause oil to get into radiator but no coolant into the oil

Is it tranny oil??