[David70]

Good idea about Mike Miller's opinion. From his "Lifetime Maintenance Schedule" (see part in bold). -



Automatic Transmission Fluid (ATF) and Filter: Variable every 15,000 to 60,000
miles

For older automatics using synthetic ATF, drain interval 30,000 miles. Old fashioned petroleum ATF, drain interval 15,000 miles
The short answer for the modern BMWs with “lifetime fill” ATF: Change it and the filter at least every 60,000 miles using ONLY the factory BMW proprietary ATF or current equivalent.

Long answer: At various production dates in the mid-1990s, which vary according to model, BMW switched to their so-called “lifetime fill” ATF in automatic transmissions, as well as manual gearbox lubricant and differential oil. The reason for this, as far as anyone can tell, is marketing and not engineering – the idea being to foster the notion of the low-maintenance BMW.

There was no explanation of what “lifetime” meant, i.e., lifetime of the car, the component, or for that matter the driver. If it was the component, then obviously anything could be “lifetime fill”. The factory’s initial position was that these lubricants never need to be changed. Then, some time later, it came out that “lifetime” means 100,000 miles. Many dealerships are now recommending manual gearbox and differential oil changes be done at customer expense every 60,000 miles. Every independent BMW technician I know recommends a 30,000-60,000-mile interval, and many recommend Red Line synthetic oils, but not for automatics with “lifetime fill” – you need the factory proprietary ATF for “lifetime fill” automatics. Older automatic transmission models, which do not have “lifetime fill” should have ATF and filter services every 15,000 miles if using petroleum ATF; every 30,000 miles with synthetic. However, the modern automatic transmissions are different. No one knows exactly what BMW’s proprietary ATF is, so no one knows if there are viable alternatives. We do know that BMW dealerships charge about $500 for an ATF and filter service, due to the price of the ATF. And that’s assuming you can get them to do the job, which is not often the case.

Bavarian Autosport (www.bavauto.com), Pelican Parts (www.pelicanparts.com), and perhaps others are now importing the proprietary “lifetime fill” ATF at reasonable prices, which they sell along with filter kits, for independent BMW shops and do-it-yourselfers.
It is risky to drain a previously un-maintained automatic transmission with high mileage, even though if it were my car I would probably chance it. Still, I have seen it happen too many times, where a well-meaning owner or technician performs an ATF and filter service on a neglected but well-shifting automatic, and then all of the sudden it starts slipping. I can’t explain it, but my feeling is the fresh ATF flushes a bit of sludge from a place where it was doing no harm to a place where it does do harm. Overfilling, underfilling, and cleanliness are also issues in ATF and filter servicing, but these should not be problematic for a professional BMW technician, dealer or independent.

BMW has backed off its lifetime fill mantra for automatic transmissions, currently recommending an ATF and filter change every 100,000 miles for the current BMW models at this writing. My inclination is to tell people to change “lifetime” ATF and filter every 60,000 miles.
However I have seen BMW automatic transmissions that were maintained break anyway. In that event, say it happens at 90,000 miles, you would like to have the money you spent on the ATF and filter change to put toward your new automatic transmission. And if I told you to spend it on maintenance you’re probably not going to be very happy with me.
On the other hand, I have seen maintained automatics last 200,000 miles. I have also seen un-maintained automatics last 200,000 miles, although both are very rare. There's just no predicting with these transmissions. When you choose to buy an automatic transmission, you also buy into the vagaries of the darn things, which is one reason technicians hate them. Whether to maintain a modern BMW automatic is up to you. At automatic transmission replacement time, we are confronted with the reality that the local transmission shop cannot rebuild BMW automatic transmissions, even those built by GM (BMW’s GM transmissions bear no resemblance to GM transmission in domestic cars). There are some domestic specialists who concentrate in BMW automatic transmission rebuilding, and you’ll see their ads in Roundel and Bimmer. However, I have no current experience with domestic rebuilders. My experience in the past is…well, the owner wound up buying a BMW factory rebuilt automatic transmission every time, and this is the course I recommend to readers – “back to the dealer.”