I emailed Martin Vogel (the fluids guy for ZF) about fluids a year ago. He wrote back that the Lifeguard6 fluid (Shell M-1375.4) is a "lifetime fill" item, meaning that it's supposed to be good for the life of the transmission. Additionally, the normal procedure for dealerships on these transmissions when doing service is drain, replace whatever part, filter/strain the old fluid, and put the old fluid back in. They only use new fluid when there has been a leak and they need to top it off.
That all being said, people have reported smoother, more solid shifts after replacing the fluid. I've read of people swearing it needs to be replaced at 100K mi, others claim 80K mi and even 50K mi. I have ~130K-something on the clock and I'm tempted to flush and replace just as good practice, though I also trust the fluids guy for the maker of the transmission, so I'm a little torn.
Has anyone else here replaced the transmission fluid in their E65/E66? If so, which fluid did you use? Replaced filter/pan? Transmission fail or problems after? Transmission problems before fluid replacement? Any noticeable difference in shifts? Full flush (including cooler) or just drained as much as possible and refilled?