A few weeks ago my friend got stuck on the A3 when his 760i decided to have a gearbox failure at around 80,000 miles. Warning messages came up on the dash to drive immediately to the nearest dealer, but instead he decided to stop the car and see if it would "reset". It didn't and it then wouldn't even move out of Park (electronically controlled), requiring it to be dragged onto a low-loader recovery truck.
The dealer said a new gearbox was needed at £6,000 and despite requests, no account was taken of the fact the car was only just out of extended warranty and had a full dealer service record. It might have just been the failure of a small solenoid valve or control board, but they don't investigate for repair and only change-out the whole unit.
Last week my sister was taking her 520i car in for service at the same delear and I suggested that she ask them to change the gear-box oil and filter as a precaution. The dealer said that it's sealed for life, requires no maintenance and they wouldn't do it.
From further reading and research on the net, it appears that these "sealed for life" units have issues which could be avoided through proper maintenance, and indeed the manufacturer, ZF, actually recommends servicing every 40,000 miles.
So, it appears that as far as BMW is concerned "life" means 3-years, until the warranty expires.
I don't know if the late-model 840 has such a gearbox, so this story may or may not be relevant.
Is it the slightly later shape ('99 on maybe?).
Any link to a service guide would be most appreciated. What fluid goes in there?
Given that my 94 car is producing 650 Nm torque 
