Thanks guys for all your comments. I've had a chance to research this online and here's what I found out about the various transmission fluids:
1. Mercedes has a technical book on this and its recommendations include those for automatic transmission fluid. They all fall under the listing on page 236.XX of their technical manual on the subject (couldn't find online). The extension XX equates to either a 6, 7, 8, 1 or 1.2 and these extensions refer to certain additives which have been tested and approved by Mercedes and which are mixed into the base 236 fluid to make it right for different transmissions. My transmission is a 5 speed automatic.
2. It is my understanding that the primary (perhaps exclusive) manufacturers of Mercedes labeled fluid are Fuchs (Germany) and Shell. The Fuchs website was very helpful in going into details for specific models and years: http:/www.fuchs-europe.de/1712.html . When you get there, go to the tab on top for oil choices and it will take you to page which will let you insert your make, model and year. Once that is done, it will show you the correct Fuchs AFT to use. For four speed transmissions there are two or three selections based on the extension applicable to your car. These are non-synthetic fluids. It appears Fuchs Titan 3000 is one of these and it apparently similar to Dexron II(D) based on their chart. Fuchs Titan 4000 is another and is similar to Dexron III(H). Fuchs 5000 SL is a fully synthetic fluid made for 236.8, which is what I believe my 2000(S) falls under. However, the site did not confirm this. It did confirm that I can use Fuchs 3353 which is new generation synthetic fluid made for the new 7 speed Mercedes transmission but is also being used for the 5 speed transmissions which would include those which I have concluded were most likely originally filled at the factory with Fuchs Titan 5000 SL. I suspect what Mercedes is now using on all 5 and 7 speed transmissions and calling their proprietary fluid is in fact the 3353, which are supposedly being manufactured by both Fuchs and Shell. I don't know if Fuchs is available in the U.S. The nuances among different brands of fully synthetic fluid are the additives they include to try and mimic the Mercedes (i.e., Fuchs/Shell) formula in their 3353 product. Apparently Fuchs is not distributed in the US to the public and it appears to be marketed only outside the US, but I could be wrong on this. It may be just as pricey if it could be purchased here, don't know but I would think not. Shell produces this product on its own label as Shell 3353 and the technical details can be found at this website:
http://www.shell.com/home/page/us-en/shell_for_businesses/oils_lubricants/automotive/app_automotive_automotive_products.html. When you go to the Shell website, you will find their 3353 was made just for the new generation Mercedes -- the new transmissions with 7 speeds (like the new E-Class) but can be used for the 5 speed transmissions as well. The Shell technical details shown for the 3353 (same link) goes so far as to indicate that the Shell 3353 was developed for
initial fill and use on the new 7 speed Mercedes. It looks like the only way to get the Shell 3353 is through your local distributor. Mine is 18 miles away. Your nearest distributor may be found at the above website with a little bit of navigating. I don't know, but would guess the Shell 3353 is a lot less expensive that the Mercedes labeled 3353. The 3353, whether Shell or Fuchs would appear from the write-ups to be a superior formula for the 5 speed Mercedes transmission models over the factory fill for the 5 speed. This is what I have concluded based on my interest in this. I don't believe the BG brand that was put in my car has the same additive or combo of additives to make it the same as the genuine 3353 article, but not sure of this since there's so little info on the BG website about their synthetic fluid. This is because it is a "one size fits all" type of synthetic fluid. That kind of marketing ploy seems very suspicious. On the other hand they seem to have a good following and reputation. Perhaps they've used all the possible additives just to cover all bases, but who knows, and who knows what the effects of that may be. Be careful before relying on any of this. I'd still only use Mercedes fluid just to be safe. Mine will be changed out this time using Mercedes fluid. I'm glad I looked into this. I was: (1) curious about whether the BG would work in my car, and (2) try and determine how this 'proprietary fluid' business shakes out. I suspected that Mercedes didn't make its own fluid. It just wouldn't make sense since, as I also discovered, they sub many if not all of their transmissions out also now.