That has a ZF 5HP24 that holds about 10.5 quarts total. I have used Amsoil ATF in these at the wholehearted recommendation of my local transmission builder, and it has never let us down. It costs 8.50- 10.50 a quart, depending on how you buy it, or who you get it from. There are a few Amsoil site sponsors here and I recommend you deal with them if that is an option for you. They are great to deal with and professional.
I understand that the newest version of Mobil 1 ATF is now recommended for this application and Maxlife ATF is listing this fluid spec on their newest bottles as well. The OEM stuff will cost you twice as much or more, but it may not be any better quality in my experience. (Perhaps better than the Maxlife).
My builder told me that these units are usually reliable and a certain part-I cannot remember what he called it -has been known to fail on occasion, but is is a known weakness of a certain part and not due to a maintenance practice. Most units never have this failure.
I drain, drop the pan, and change the filter. Then I use about 12 or 13 quarts total to completely flush the rest of the old fluid out (via a cooler line) and refill. It would be a whole lot easier to do if BMW had designed it with a dipstick tube to pour the fluid down into, but you must pump in every blessed drop to do the job.
