This a great DIY Thanks for sharing. As you indicate we can only drain a part of the transmission fluid capacity. I am not 100% sure for the X5 but the following statement was true with most BMW's automatic transmissions in the past. Most of the transmission fuild is in the torque convertor which cannot be drained without dissassembling the transmission. The entire fluid can be refreshed though by flushing the transmission. To do that the fuild needs to be drained several times with new fuild and by running the car for few minutes each times. That will consume more than the full capacity of the transmission but it is a good way to replace the entire transmission fluid.