New fluid changes the operating environment of all the friction surfaces of the transmission. This can cause accelerated wear of those surfaces resulting in clogged filters or fluid passageways. The greatest danger is the possibility that a non-compatible fluid was used accidentally. With today's synthetic based fluids and electronic throttle/shift controls, mechanical wear has been reduced significantly. As long as the transmission has not been overheated, it is not necessary to change the fluid.

My background includes a degree in automotive engineering from General Motors Institute in 1978, with lab experience with the original lock-up convertor turbo-hydramatic transmissions, and engines families including the Cosworth Vega, Buick 3.8L turbo, Pontiac Super Duty 455 and turbo 301. Along with ten years at GM assembly plants, I spent another eleven with Japanese and German HVAC Tier One automotive and truck suppliers.

Count me as one who doesn't change fluid anymore.

2002 X5 3.0 243,000 miles
2004 325i 108,000 miles