The X5 4.4 transmission holds a total of 9L. BMW says the fluid never needs to be replaced but for all we know they might consider 100k the life of a vehicle. Some say that changing the transmission fluid will "shock" the transmission because it adapts to the fluid or something. A simple drain and fill through the plugs on the transmission oil pan will change about 4L, 45% of the fluid, and that's better than nothing - which is what BMW says to do. Changing about half the fluid should be a good compromise to satify the critics and skeptics. As long as you re-fill it with the same fluid that's already in there, this should re-vitalize the transmission fluid.
I am doing this as part of my 60k mile maintenance.
There is also a transmission filter that can be replaced but that requires dropping the whole transmission oil pan and I don't believe it's necessary at 60k miles, especially when it's supposed to last forever.
Here is a quick DIY for changing the Automatic Transmission Fluid for a 2001 X5 4.4
You will need 5L of ESSO ATF LT 71141 or VW G-052-162-A2 Transmission Fluid and a fluid pump. (Available at any VW/Audi Dealer)
1. Lift the car with ramps or put it on jacks stands so that the oil pan is level with the ground. Put a level up to the tranmission oil pan if you're not sure.
2. Make sure the engine/transmission is cool and the car has been off for a while before proceeding, this will ensure all the oil is in the oil pan. Place an oil pan underneath the filler plug.
3. Remove the filler bolt with an 8mm hex socket. You may need a breaker bar.
WARNING: A lot of oil will come out once you remove it.
4. After the oil is done draining from the filler hole, move the oil pan underneath the drain bolt and remove it with an 8mm hex socket.
Neither of the bolts have crush washer so you don’t have to worry about replacing them.
5. After the oil is done draining from drain hole, re-install the drain bolt and torque to 30 Nm (23 lbs-ft).
You should have roughly 4L of oil drained out of both holes.
6. Pump the new fluid into the fill hole until a steady stream begins to leak back out, it should take about 2L
7. Loosely re-install the fill bolt just enough to stop the oil from draining.
Take caution in the next couple steps.
8. Make sure the car is level to the ground. Start the engine and turn on the air conditioning to increase the idle speed and ensure that all oil passages in the transmission are filled with oil.
9. Apply the parking brake. With your foot firmly on the brake pedal, shift the car through each gear position, pausing briefly in each gear.
10. WITH THE ENGINE RUNNING and the gear selector in Park, remove the filler plug on the oil pan and begin pumping in more transmission fluid until a steady stream leaks out. It will take a little more than 2 more liters.
WARNING: The exhaust will get very hot and is on both sides of the transmission oil pan, be careful not to touch it!
11. Re-install the fill bolt and tighten to 35 Nm (26 lbs-ft).
12. Only after you’ve tightened the fill bolt, turn off the engine and you’re done.