This is an older thread, but a good one.
It's always interesting to see what BMW puts in their Service Training Literature...

ST057 Electronic Transmission Control (January 2003) / Electronic Transmission Control P2

Transmission Fluid Information
Transmission Fluid (Oil)
The automatic transmission provides filtered, pressure regulated hydraulic fluid for all of the transmissions functional requirements. All BMW automatic transmissions are designed to operate with specific fluids. Use of non-approved oil will cause malfunctions and irreparable transmission damage which is not covered by BMW warranty.
The transmission fluid provides the following functions:
• Lubricates mechanical components (planetary gears, bearings etc.).
• Removes heat and transfers heat to transmission cooling system. (Heat Exchanger).
• Removes debris and contaminants to sump and filter when circulated.
• Provides a transfer of kinetic energy in the torque converter.
• Allows hydraulic operation of mechanical components (clutches, brakes) via control of the valve body.
Also, transmission fluid has various properties to prevent oxidation and breakdown from heat and friction. Each type of transmission fluid has properties specific for each transmission application.
Fluid level is crucial in the proper operation of an automatic transmission. Improper fluid levels will cause improper operation and eventually irreparable transmission damage.
Improper fluid level can cause:
• A low fluid level can cause an interruption in oil flow during fast acceleration or hard braking which can cause gear shift malfunctions and noises.
• An excessively high fluid level can cause the rotating mechanical components to paddle in the oil. This produces foam which introduces air into the hydraulic system.
• A low fluid level can also cause transmission overheating causing premature transmission failure.

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Transmission Fluid Checking Procedures
Transmission fluid checking is accomplished using the DISplus or GT-1. The DISplus or GT- 1 is used to monitor transmission fluid temperature to insure the transmission is not over or under-filled. As with most other current ZF transmissions there is no dipstick, the fluid level is checked and filled at the fill plug. The location of the fill plug varies between transmissions.
Transmission fluid should be checked between 30 and 50 degrees Celsius (unless otherwise specified). Use the DISplus and/or GT-1 to determine transmission temperature. The transmission temperature information can be found in the diagnosis section under Service Functions.
Proper procedures for checking and filling transmission fluid can be found in BMW Service Information Bulletin B 24 01 98. When checking transmission fluid, observe the following items:
• Transmission in Park
• Parking brake applied
• Engine Running
• Vehicle level
• No engine load
• Trans Temp 30-50C
• Observe correct drain plug torque
• Use correct fluid
When replacing parts on transmissions that use lifetime fluid, drain fluid into a clean container and reuse.

[PHOTO]

Transmission Fluid Application
There are numerous types of transmission fluid used in BMW transmissions. With the exception of the early transmissions (4HP22/24, A4S310/270R and the A5S310Z in the E34) all current BMW transmissions use “Lifetime Fill” transmission fluid. There is no maintenance required for these transmissions. It is important to use the correct fluid. Incorrect use of the transmission fluid can cause non-warrantable transmission damage.
When performing repairs on transmissions with lifetime fluid, it is important to drain the transmission fluid in to a clean container for re-use. New fluid should only be used for transmission replacement and for topping off after repairs.
Also, transmission fluid level is vital to the proper operation of the transmission. Refer to BMW Service Bulletin B 24 01 98 for proper fluid level checking procedures.
When servicing or repairing BMW automatic transmissions, refer to TIS for fluid capacities.
For fluid types refer to the “Operating Fluids Manual”.

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There was a SIB notifying the change from "lifetime fluid" to 100,000 mile replacement, but I can't locate it at the moment.

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2010 X5 3.5d: SOLD, but miss the torque...
2004 X5 4.4i: SOLD, but served us well