BMW says the fluid lasts a lifetime, whatever that is. I have changed the trans fluid and filter every 50,000 miles since new. In spite of mods, other than 1 sensor, I never had a problem with trans in 118,000 miles. I look at color and smell it but only for an indication something else may be going on that I need to look into.

I only use OE fluid. IMO it is part of the design criteria even though some other fluids are compatible. I pull the pan and replace the filter. With pan back in place. I add fluid until it starts to drip out the fill hole. I replace the plug and run the engine until the transmission shows a reading with infrared temp tool of about 110 degrees F. Then pull the plug and add fluid until it starts to run out again. I replace the fluid and drive it 50-100 miles or so and drain and replace the fluid again since the torque convertor doesn't drain when the transmission is drained. Doing it twice increases the percentage of new fluid. Quite often if you are already having a trans problem changing the fluid will not help.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas