Crazy they have no fill pipe or way to measure the level.
I used the way expensive BMW fluid and something I rigged up to plug into the the drain hole.

I measured the fluid that came out and sought to put as much back in. I was probably a quart less. A private bmw mechanic told me later about some complicated procedure of running the motor trans and it taking in the last bit.

The fluid was dark brown and clearly needed changing at 100K and 7 years on the X5. I have also changed the fluid in the differential, it was a pain for access I had to buy a special wrench I found online for $30.

I have 140K on the car no sign the transmission isn't working well so guess I did the job well enough. This is a job to be very careful about, if you end up with a trashed transmission that might cost more than the car is worth.