What is the official (or unofficial) MB recommended interval (if any) for transmission oil changes?

 

Don't know about the s55, but it's recommended at 40k for the cls55 that has the 5spd auto

 

My service booklet from 2004 doesn't mention it at all. My dealership recommends every six years or 100,000 mi for all automatic transmissions after 1997 (I don't know where they got this number). I just checked the newer (2006 and later) manuals, and they specify once at 40,000 mi (not every 40,000 mi?). IMO, every 40,000 mi or every other B service sounds like a safe bet.

Here's a link to all the service manuals.