You can pay for a LOT of oil changes for the cost of one $400 oil analysis. Oil is relatively cheap, and if you do your oil changes yourself, as it appears from your other messages that you do, all it costs you is some of your time and a few bucks for oil and a filter. I would say frequent oil changes are cheap insurance against engine wear and failure, and if you do it more often than really necessary, what's the harm?
Further to that, you have a diesel, and Mercedes or not, diesels have their special requirements. Because diesels use the heat of compression for fuel combustion, the high pressure in the combustion chamber causes combustion gases to blow past the piston rings and into the crankcase. Nothing you can do about that. This is what causes your oil to turn black after 30 km. For this reason, it is probably more important to change oil in a diesel more often than a gasoline engine.
[This message has been edited by TimBru99 (edited 09-17-1999).]