With oil changes done at Mercedes recommended interval and with only 35,00k, your engine is not gunked-up inside.
If you scraped the bottom of the oil while taking the oil sample, it is quite possible you picked-up more debris than you should have.
Diesel engine are not significantly harder on oil than a gas engine.
Diesel engine will generally produce more soot and have more fuel contamination, but remember, diesel engine do not have high rotational speed to deal with, (RPM) which tend to foam and heat engine oil in a gas engine. In addition, diesel engine generally contain more oil than a gas engine (not sure if it is the case with Benz's.)
Whatever oil you use, I would suggest to discard it as per Merc. prescribed interval AT THE LATEST. If those merc. engineers prescribed oil change say at 12,000mi, be assured they have reasons for it, whatever oil you use.