I don't know the oil capacity of a 300D, but have you checked the dipstick?
Diesel engines require an oil with an American Petroleum Institute rating of CI-4. It's right on the label in plain view. The commonly available national brands are Chevron Delo, Mobil Delvac, and Shell Rotella, and the common multi-vis grade is 15W-40.
Brand name isn't important. The important issue is to us an API CI-4 oil. Gasoline engines use API rating SL.
If you don't have an owner's manual, you should try to locate one. A Mercedes Dealer is probably a good starting point.
It won't hurt to let run the SL oil for 3000 miles, but make sure you get a CI-4 at the next oil change, and make sure the next place that changes your oil has a suitable CI-4 oil and you tell them the proper capacity.