TRANS FLUID : search threads here, but my strong and painfully acquired advice is do not change it at this aged point if it has not been done regularly (operative definition of "regularly" I really can't say), but I properly (stress = properly) changed my never-before-changed fluid at aprx. 90k miles in about 2016 with reputable and specifically (and uber $$$) BND fluid and new filter and seals etc. and had no-zero-zip-zilch-nadda trans problem symptoms prior to change and within 200 miles of changing fluid the trans went big boom and imploded, totally dunzo. Granted trans / torque converters are famously problematic in this car, under-beefed for the power, ancient old school viscous coupling design etc., and at 90k miles and aprx. 13 years old as of 2016, it surely wasn't going to last forever but there are many untouched stock trans' on record going well past 150k mi.s I believe, but as was best and most succinctly explained to me by multiple and credible trans experts including some very familiar with RS6 trans and its service, replacement etc, over time (stress = gradually over time) debris / particulates etc. can be introduced to the fluid from wearing clutch packs etc and as that gradually happens, said particulates not otherwise trapped in filter can find their way to depositing and anchoring into nooks and crannies of the case or wherever and be harmlessly out of the way. And as well, as explained to me, over time the detergents, or similar ingredients if that is not the precise right word, that come in the trans fluid, wear down in their effectiveness / potency and allow those now harmlessly deposited particulates to remain anchored in their nooks and crannies and out of the way from circulation with the fluid. Brand new fluid will have fresh strong detergent components, and perhaps the full flushing of the old fluid out and new in can play a part as well, but it is not uncommon for those once safely deposited particulates to dislodge and start circulating with the new fluid and apparently as described "there are many critical fluid passageways within the trans that are no bigger than the lead of a pencil and if any of them become clogged its sudden death. So take it for what its worth, but that was my personal and painful experience, and I read similar to what I just told you from multiple sources, but I didn't listen and insisted on being needlessly - and indeed harmfully - proactive in "caring" for my vintage and rare cool RS6 I really liked, and ended up facing a full trans replacement as a nearly immediate and direct consequence. Trans replace / rebuild = involved and expensive full engine -out undertaking I don't care if you do it yourself or pay someone else or whatever, not something I'd think you'd want to do with your new-to-you car. My strong advice, if it seems fine, or even mostly fine, and if you can't verify "regular" fluid changes over car's life, and maybe even if you can, don't mess with it. Other bad part imo and in my personal psych's case, whenever I break something on any of my car toys, I always try to fix / replace it better than it was new to lessen the pain and anguish of having to do the repair in the first place, and short of a manual swap (read = uber uber reinvent the wheel type of involved and expensive and makes what MIGHT one day be a valuable / semi valuable rare car no longer stock in very substantial ways) there really is not much that can be done to make the trans better than stock new, few small small mainly longevity upgrades from my research and TCU upgrade to i.e. MTM as you mentioned which I did do with my repair. In other words, if your trans is working fine now and you blow it up trying to "take care" of it, and have it replaced, you will be no better off from a performance / driving perspective than you were prior to blowing it up, except you'll have a newer trans in place of the one that had been working fine anyway.... one semi mitigating factor to the root-canal-like pain of the engine-out procedure is that it will provide optimal opportunity for addressing whatever other sundry issues you might have with such an old car, many of which are likely total "good luck reaching that" type of propositions when that engine is in place, stuffed in there like a fat lady's foot in a high heel shoe a size too small, as that engine is. Enjoy the car and leave trans alone if its working fine, or if you insist on messing with it as my dumb non-listening ass did, at least drive it some first before you mess with it so you'll catch the bug and be motivated to fix it pronto after you blow it up.