start by replacing the filter and about 3 quarts of Dextron III fluid that will and up coming out. After this you can replace about 3 quarts every oil change and a new filter on about the 3rd change. This way you wont loosen up too much crud on one attempt and you will eventually end up with clean fluid. If you have the whole trans flushed at once you run the risk of damaging the valve body from too much crud floating around.
95 E34 530I V2.37
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Those who make peaceful revolution impossible will make violent revolution inevitable.
John F. Kennedy