As VA300SD said, you do have to give the car a good hard run every so often to keep things cleaned out (i.e. carbon). I give my 300SD an "Italian tune-up" just about each time I get on the freeway with it, and when I travel back to Cleveland on the weekends from Cincinnati (about 250 miles), I will try to keep it at a constant 4,000rpm traffic permitting, that is. It doesn't hurt to use a bottle of Red Line Diesel Catalyst every now and then as well. I use Castrol GTX 10W-40 in all of my MBs, but tried Castrol RX Super 15W-40 in my 300SD on the last oil change. It is formulated for diesels and does meet Mercedes-Benz engine oil standards, but during this cold weather I think I am going to go back to GTX 10W-40. In addition, I notice that the car runs cooler with 10-40 in it versus the 15-40 on those hard highway blasts. Hope this helps!
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'93 300E 2.8