When I got my 2000 E53 M62 back in 2012 it had just short of 44k miles on the clock. I had the gearbox oil and filter changed immediately as I don't do "lifetime" oils! I understand they can only change 3-4 litres of the total, so some of the oil is now over 15 years old and has now done over 60k miles. The gearbox is absolutely fine (touch wood) but how often do folk think the oil should be changed (whatever amount you can get at to change, that is)... I'm inclinded to think to do what I can every 30k miles but maybe not the filter every time...?