To check transmission fluid level:
1. Vehicle must be on level surface (or raised in air level).
2. Transmission must be at proper specified temperature (I believe the temp range is between 35 and 45 degress celcius. Monitored via INPA or other similiar program).
3. Shift transmission through gears pausing at each gear (including reverse).
3. Fill plug removed. Fluid is at proper level when fluid is at bottom of fill plug hole and transmission within proper temp range and in park.
BMW specifies "lifetime" fluid for all its transmissions and defines lifetime as 100,000 miles. Therefore, it is a good idea to service (drain and replace) your fluid every 50,000 miles. If you're unsure of the last service, replace the fluid as soon as possible. Doing so will extend the life of your transmission. There are many options for replacement fluid. Choose the best you can afford and be sure it meets BMW's current specifications for your vehicle. To determine the fluid that belongs in your transmission, check your owner's manual.
The correct fluid type = Pentosin ATF1 or ZF-Lifeguardfluid6 AT fluid - avoid the dealership AT fluid as very expensive @ upwards of $36.00 per liter bottle!.