Originally Posted by JCL

If you want to change your fluid to be as conservative as possible, that is fair. It is your transmission, and your call.

However, you are taking the guidance of the component manufacturer over the guidance of the system manufacturer.

The document says that very high fluid temperatures will age the fluid prematurely, and I fully agree. It then goes on to say that various types of severe service (as listed above) can cause such high temperatures, justifying early changes of the fluid (every 65,000 miles or 8 years). One thing to keep in mind is that BMW addressed the high temperature issue with the cooler that they designed in to the vehicle. We can discuss whether or not that cooler is sufficient I suppose. I would be more worried about high ambients, trailer towing, etc, if that cooler wasn't there. It is because of the total system design (the cooler, the engine software that backs off engine power momentarily to soften the shifts, the torque converter lock-up clutch strategy that reduces heat from the torque converter, etc) that I would look to BMW's guidelines as more relevant than ZF's.

It would be interesting is to see what actual fluid temperatures climb to compared to ZF's application guidelines. If the BMW cooler isn't able to maintain them within ZF guidelines, then the ZF fluid change recommendation would be very relevant. If the BMW cooler is able to maintain fluid temperatures to the design specification, then there is no severe service recommendation applicable.