Peter Fearing wrote, "Did I miss something? I never changed the trans fluid in my '(97 ) C280, 100K miles and the trans worked fine. Think of it. Most trans changes only remove about 25 to 39% of the initial OE fluid. Trans are closed systems, so what happens to the synthetic fluid (other than a possible leak) that would require a fluid change?"

If you drain your fluid it will be chocolate brown. Matter of fact, a friend of mine just did a trans service on his '98 SL. He reports chocolate brown fluid. He thinks he will either do future services at 30 K or 50K. My trans service at 109K, when I bought the S320, was not chocolate colored. However, the prior owner was smart and had it serviced at 65K.

So the fact that your transmission is fine now has nothing to do with whether it will fail next week. The 722.6 is an electronically controlled transmission. It's expensive to replace. A transmission service at a independent shop using the correct special MB fluid should be less than $250.

Not certain about your particular model, but the 25 to 39% figure you quote is dead wrong for MB with Torque Converter drain plugs. At some point they eliminated the TQ plugs, but my '97 S320 still has one. Transmissions may be closed, but the friction material does wear and gets suspended in the fluid. It then coats the bottom of the pan.

By the way, it's also a good idea to change the differential fluid, the coolant, and the power steering fluid, and brake fluid.

Steve