I work in a transmission shop, ( 15 years ). I can tell you from first hand experience that every time we change the transmission fluid on a bmw with ZF tranny (4.4L) or GM tranny (3.0L), the fluid is almost BLACK in color, dirty and has a burnt odor. When it comes to changing transmission fluid on any transmission, including BMW, Mercedes or any other brand, the same laws of physics, of chemistry, of tribology (lubrication), of heat transfer apply. The fact of the matter is simple : transmission fluids get old, dirty, oxidized and contaminated, with time and usage. So change it and change it often (every 30,000 miles ) if you want longevity and like- new performance for your transmission. Also change the filter too. SEEING IS BELEIVING.