the drain stuff is a given to me now on these units from what I have come upon. More on that below. But along the way it also may be part of getting at a specific component buried inside the tranny likely to cause issues for at least some of us longer run:
See picture below. 105K miles. It is the seal between the Mechatronics unit (a.k.a. valve body) and the guts of the tranny case/unit. Likely faulty one on top; new OE on bottom. Accessed by dropping the Mechatronics unit, which is right there in plain sight once the pan is dropped--it spans most of the pan size.
I searched high and low and found Audi related 6 speed tranny info slim to none. To be honest, I was disillusioned in the quality of some info in a tranny fluid post a few months back so figured I would just move on and let others tell their competing folklore while I pulled out the real tools and got into diagnostics and better research. I also think the board should move past the old "hoot" TSB that gets dredged up from time to time and get to more current info.; these cars have a lot more years and miles than when Audi, BMW and Jag issued their first versions of that TSB, for cars still under warranty and pretty young.
There is a lot more info about the tranny, and in turn the failures of this same seal, found for BMW's, commonly in their V8 models. Here is a link to one repair shop that spent the time to post a bit of info: http://www.autohausnc.com/zf-transmission/
The pictures of the same seal in their repair are even more sobering. And there are more to be found if you Google for images--as in, not a one off fluke most likely. Also to be clear, this is not an easy job--all the way up at a 9 or a 10 on the 10 point difficulty scale. The pan drop is already a pain, at least on the W12 and the S8 where the exhaust also has to come down and various other braces and shielding cleared away.
Along the way FWIW that bears on both ZF 6 speed tranny issues, I got this feedback from a ZF distributor and rebuilder I have used and trust over my last two higher powered Audi's :
"We see problems with solenoids in these units that can cause some of these codes, and wear in bores of vb, but we also see bushing wear issues. The B drum in particular and sometimes E are dependent on bushings. The B drum is located relative to the part it seals on, by a combination of 4 bushings, so a little wear in each adds up to a lot of motion between the drum and the part it seals against. The E drum uses the clearance between the shaft and bushing as "controlled leak" and wear of bushing can cause a pressure loss to the drum. To make matters worse the European Union made laws requiring "greener" materials and lead etc has been removed from the bushings in these transmissions."
Read it carefully. My conclusion was, among other things, that cleaner fluid is that much more important. And again on some of the opinions or ideas floating around that I think are now arguably obsolete, notice the subtle reference to no more lead in tranny parts. This is an experienced manager at a name ZF rebuild distributor and shop with field experience inside these units which he is describing. That means concepts of letting the tranny just settle in from older historical experience have been upended by materials changes that no longer have the inherent lubricating properties they once did--just like a whole generation of old motor heads found with head rebuilds and valve seats when lead went out of gas in the 70's.
I can post more info., pictures and links in a separate meaty thread if there is genuine interest. In my case, I had a 6-5 shift down flare where it would drop to 4th and could kick to SAFE mode if I didn't back off fast. It developed pretty quickly, maybe over 5K miles from where I first heard a faint hoot type noise last winter on some higher gear kick downs at light throttle to the full blown 6-5 shift issues. I could sort of work around it with S mode or paddle shifting, but found myself reflexively starting to baby it/hesitate to make moves where the tranny needed to perform 100.000%. Needless to say, I don't plan to own a W12 and then drive it like a proverbial church lady. With full fluid and filter change per the complete 3x cycle in the TSB AND the code reflash AND full adaptation as my first go at it, disappointingly NO fix/change on mine. As in, with ALL steps of the "hoot" TSB done "by the book," no joy.
My exhaustive fluid drains never found more than dirty fluid--no metal bits, clutch stuff etc. whatsoever that are the bad news revelation sometime found w/ a pan pull alongside some tranny issue. That was my cue it was probably NOT in the conventional wear parts. I guessed solenoids and changed them in this same follow up job, but it could well be it was just the seal in the picture. The complete solenoid set was about $450 via ZF, so it didn't break the bank anyway. Other BMW posts suggest the seal can be tied to pretty harsh 1-2 or 2-1 shifts (clunks) and a variety of other things. They often do at least selective solenoid swap outs too. With a new set of solenoids and the seal change and a fresh adaptation, it shifts literally like new (I bought it back at 15K miles). I have enough miles on it now since I did the work to be definitive; night and day just as soon as the adaptation was done and the shifting settled in to its renewed form. Glad I got to the bottom of it with a definitive fix for mine.