I changed out the fluid in my '00 c280 at 60k miles with the 722.9 (3353) fluid. The fluid that came out of the transmission was brown and murky. The new stuff is pretty and purpley.
That was 3,500 miles ago, so far, no problems to report.
Like Ethan said, the upshifts seemed slightly smoother at first, but could just as easily be in my head.
Changing out the used up old fluid with either the 722.6 or 722.9 specified fluid would probably yield the same result here.
Having said all that, I don't think you can go wrong with using the new 722.9 fluid in the 722.6. In fact, it was actually $.50 less than the 722.6 fluid at my local dealer.
