I think for them lifetime means to always have the car inspected and serviced by BMW on a regular maintenance schedule. I owned an E 34 , it had 295K and almost 19 years on the original tranny oil & filter, shifted fine without problems before I sold it,I was the second owner and car had been serviced only by BMW. They engineered the transmission oil and cooling process of the oil to be lifetime. But still you never know what previous owner did to it. Maybe he changed a failed radiator, drained some ATF and never top it off. Maybe he drove the car on manual mode exceeding the speeds recommended for each gear..you never know!
I do believe that is lifetime oil. Under normal driving conditions and car under a BMW maintenance ! My 99 740 has 149 K on the original oil & filter and shifts perfect! I'm thinking to change it because sometimes my driving is aggressive and exceeds their recommendations. (4000-5000 RPM and I don't stop until I reach 120+MPH) Then comes that feeling that you get driving a BMW! The Ultimate Driving Machine!