As another personal report, both of our 300E's have run (almost) exclusively synthetic (either Mobil One or Castrol Syn) on approximately the 7500 mile change interval. Both cars followed the prescribed maintenance intervals religiously while they were under warranty, but as the cars got older, this schedule was followed less and less closely. Started on 15W50, then moved to 5W50 after a few years, then ran 10W30 for a couple years (before I realized it was not a MB-approved viscosity), and am now switching back to 15W50.
1987 300E (237k) - Had a valve job done about 8 years ago around 140-150k miles. Valve seals last year (about 215k), now uses about a quart every 1200 miles, fouls a couple plugs, and I have intermittant lifter noise until the engine fully warms.
1991 300E (185k) - Valve seals this past year (about 175k), uses a quart every 800-1000 miles. Has not had any other engine work, as far as I can remember. I think it is also fouling a couple of plugs.
I believe there is value to what Larry says about oil change intervals.