Even in park the oil will still come out as the transmission and torque convertor will push the oil through. I have never let it run dry before as the manuals do not recommend it. Remember the fluid is there to cool the trans as well as operate some valves etc. The way i do it is cut the hose drain 1-2 litres, then add 1-2 litres....untill all clean. Then i stick a Magnefine filter into the line. The manuals all state the possibility of air getting stuck into the and having to use a compressor to clear etc, so by adding fluid each time will help to reduce this. The last think you want in there is air as there is nowhere for it to vent to....just a thought, great write up anyway.