Hi Bob:

Welcome to the world of the M-class. I have something like 119k miles on my M so let me give you my data point on FSS intervals.
First, the engine sump has about twice the oil in it as a standard
engine so, in terms of diluted dirt, you are only putting about 6k miles per oil change which is only twice that of the '3k mile' rule of old. With the tight manufacturing tolerances and great oil available today, the 12k-13k normal FSS intervals are no big deal.

If you are worried, have your oil tested. I do and the test house always tells me to keep the oil in longer. I started with Mobil 1 10W-30 and switched to a heavier grade after MBs letter went out. I now use Mobil Delvac 1 SAE 5W-40 which is a universal oil formulated for diesel but also suited for fleet gas use. The choice of oil is an exceptionally emotional topic as a simple search of these archives will reveal. There is a list of MB approved oils at:
http://www.whnet.com/4x4/index.html
under 'Shop Talk'.

May I suggest that you get copies of the MBCA 'The Star' for May/June 1998 and July/August 1998. These two issues have DIY instructions for the common maintenance items such as oil and filter.