On my 03 4.4i maintenance manual it said:

Replace transmission fluid every 100,000 miles
Replace spark plugs every 100,000 miles
Replace break fluid every 2 years.

Nothing about the transfer case and/or diff. I bought the oil for these and will get it changed out sometime next week.

Mine says the same. When I asked the dealer advisor why I needed to buy an entire BARREL of the ZF fluid to do the drain/fill on my X he replied that BMW did not update the manual to show the revised data and the transmission tag stating "lifetime" supersedes the manual info and therefore it is only sold in high volume. Was told by 3 different Florida dealers that the min volume sold is a barrel!!!

I'm in the camp to replace the transmission fluid periodically. Had the same "lifetime" fluid issue on my 1999 E320, and later MB issued a service advisory to dealers to change it every 60k miles. I've had my mobile mechanic change the tranny fluid on my 2005 X5 every 3 years.

Rather than beat a dead horse, I have a question. Early on in this thread someone posted $3500-5000 for a brand new dealer installed tranny. I was under the impression that the dealer would charge well over 10K for a brand new tranny, and that a rebuild (not from a dealer of course) would run in the 3.5-5k range. Has anyone had an actual quote or actual dealer new tranny installed for $5k or less?

The one from the dealer would be a rebuild as well. No one sells a new transmission, unless you get a new car with it. You will get a rebuild replacement under warranty even if you just drove off the lot.

The distinction is that a dealer sells an exchange rebuilt unit. They take your old transmission in as a core. The one you get has been reconditioned to new specifications. I would expect the price to be close to $7000 depending on the model.

When you go to a shop that rebuilds your transmission, it is usually less of a rebuild. It is on your own transmission, not a different core. They will focus on what failed, whether the valve body or torque converter or clutch plates. They are repairing worn parts, not necessarily returning the transmission to new specifications. That isn't a bad thing, rebuilding it can be cost effective and a good decision, but comparing it to the dealer exchange unit is apples and oranges.

I wouldn't define towing a trailer as extreme duty. The vehicle was designed to tow that weight without any additional modifications other than a hitch installation.

All transmission fluid has friction modifiers that impact clutch engagement right at the threshold. It is the primary difference between 10w hydraulic oil, and transmission fluid. The issue is that all friction modifiers have different characteristics, matched to the clutch engagement characteristics. If you put the wrong fluid in, the most likely result is poor shifting. Unless you are slipping the clutches enough to burn them, it will probably have little impact on transmission life. It will affect the quality of the transmission's operation however. If the slip is extreme, you get what RRPhil characterized as disastrous consequences. If you have made it a few thousand miles with no impact or noticable degradation of shifting, on non-spec fluid, I wouldn't worry about the friction modifiers too much.

The second issue is that purchasing a fluid that has not been tested to the manufacturer's standard means that you have no idea how long it will last. What are the quality of the anti-oxidants, the detergent levels, and so on? Who knows? All you really know is that it is wet, red, and of about the right viscosity.

I just don't see the reason to take the chance when the correct fluid is readily available.

I disagree that if you did a teardown of a transmission that failed when the incorrect fluid was used, you would have to find a failure mode that had never been seen before. You would have to demonstate a correlation between the observed failure mode and the properties of the fluid. As an example, if a normal failure was a bearing surface failure at 100,000 miles, and the same failure happened with non-spec fluid at 10,000 miles, it would be the same failure mode but much earlier. I haven't been to aircraft school, but I have taken applied failure analysis. Automotive and heavy equipment, in my case.

In the aircraft industry, I would think that using the correct replacement parts is a vital part of the certification process. Do aircraft maintenance technicians buy their supplies from the lowest bidder, not worrying about specifications? Or are they required to use things that actually meet those specifications according to independent tests? A rhetorical question.

How the see it in the birthplace
 

if you have never watched this regardless of which side of the fence you are on with the tranny argument ( i always learn a great deal by the way) this video is worth a glance. thumbs up if you like it.
:thumbup:

BMW E38 Transmission service by ZF Germany - YouTube

Thank you both for the clarification. Any idea what terms (years/miles) of warranty comes with a dealer installed rebuild, assuming the original warranty has expired?

200k miles
everything is original

24 months. Confirm that with your dealer, but that is what BMW advertises for remanufactured parts.