As regards the recent letter from MBUSA, it is specifically referring to 1998 and later vehicles that have the FSS (Flexible Service System). MB diesels of this vintage run on direct injection technology and, thus, do not have pre-chambers. As such, the levels of soot accumulation over time are GREATLY reduced. For this reason, running longer drain intervals with synthetic oil makes sense.
For indirect injection diesels, which includes all MB diesels prior to '95 (with the exception of the '95 S350-OM603.97), synthetic oils will assist with cold weather starting and WILL reduce engine wear over time. However, soot accumulation and oil contamination will still occur. It makes sense in this case to change the oil more frequently. I use Mobil 1 and change every 3-4000k mi.
All this, of course, is IMHO.
Happy motoring...