[surfwooder]

You don't give any information about your Z. Have you ever changed the brake fluid, transmission fluid, cabin filter, or any other fluids and filters since you owned the car? Brake fluid will remove moisture from the air, and deposit it the lowest level of the brake system, like the calipers. There it will rust the inside of the brake pistons. BMW touts the auto transmission fluid as lifetime. I say it will last the life of the transmission, or until you have to change or rebuild the transmission, then it will be changed. I just changed mine on my 2009 Z4, I bought last year, the fluid resembled lumpy oil. I drove it today and it shifted much better and smoother.