I just replaced my transmission oil last week. I purchased the oil filter and pan gasket for less then $400 with tax. I did not want to have issues with the trans or with BMW NA saying I used the wrong fluid. The BMW part number is #83 22 0 142 516. The transmission fluid is also available from BMP Design in one gallon jugs. I used 5.5 liters. The dealer was more then happy to sell the fluid to me, and the service department recommended changing the fluid at 90k miles. I have a photo of the five gallon container they sold me if you need.

BMP design : BMW Automatic Transmission Fluid (M-1375.4)

The change over was easy and I have put close to 750 miles on it with no issues.