Oh yeah - here's the oil analysis results on my blue 87 300D, and my 84 300D (PDF files) :
http://www.meimann.com/docs/mercedes/oil_analysis_87b.pdf
http://www.meimann.com/docs/mercedes/oil_analysis_84s.pdf
I get the 'extended drain' test kit from Snider Petroleum in Sumner, WA and test at every oil change:
http://www.sniderpetroleum.com/oilanalysis.html