Transmission filter & oil change
I did the same to my trans. 78000 miles. I used a trans kit from ECS tuning filter and shell oil kit. I used ZF filter/pan a little cheaper than BMW one the same part number. I used the shell oil. No other oils seemed to mix with it. Being the ZF trans costing alot of money to replace $6000 + new( I couldn't find a rebuild kit) using the recommended oil would be a smart choice.
If you want to try to remove the most oil from your trans. Without using the flushing method. Get your car on jack stands or lift. Remove your drain plug and let the trans drain over night in a pan. The torque converter will drain back into the pan too. about at least half of the volume. also the cooler lines and clutches will drain some too. I did this and just about used all the 7 liters of oil that came in the kit. Yes I had a leak in the pan area so I couldn't use the oil removed as a gauge. I did put more in than was taken out.
The best way to get your oil level right is after you install filter/pan and torque the bolts. Is fill the trans until oil comes out the fill plug and loosely install the plug. start the car up and run the trans through the gears with foot on the brake pedal. then shut the car off remove the fill plug and top off the oil level again which could be about two or more liters. loosely install the fill plug and start the car again and run the trans through the gears again. then with car still running check the level again and adjust again if needed. loosely reinstall the plug again. and let the car run until the trans temp is up to the temp written on the trans pan. I used a Non Contact thermometer pointed it at the pan in several locations to get this temp. or if you have device that plugs in the OBD II port and interfaces with the trans ECU and uses the trans temp probe to measure fluid temp. I waited until oil temp was at the upper part of temp scale that is written on the pan then I removed the fill plug and let the excess oil drain out until it was dripping slowly out the fill plug. Then reinstall plug & torque.Take the car for a test drive and warm up trans to operating temps. then check for leaks and if none found you should good to go!!
The most important thing you can do is to get the proper oil level at the right temp. If the Oil is to cold at fill plug level can or will cause shifting issues when oil does get up to operating temps. The oil will expand more than was intended. which could aerate the trans oil and causing shifting issues(Delayed, slow, harsh shifting and or slipping can happen). don't over fill.
I hope I was able to help. I breezed through the posts seem to be a little confusion on this. Also hopefully I didn't repeat what someone else said.
just trying to help