Basically, it means leaving the oil in longer than the car manufacturer says you should. I think most older Mercedes diesels owner's manuals say to change the oil at 5kmi in normal service, or 2.5k under severe service, or something like that. However good synthetic oils are not close to worn out at 5kmi and changing it is wasting money. Of course you can continue changing every 3kmi (with dino or synthetic) and be safe, but if you drive a lot, this is a real nuisance - and not great for the environment either. :p

The problem is, you need to do an oil analysis or two at your estimated extended interval to see if it's OK. The usual limiting factor is soot levels. The limit MB says to stay under is 2%, regardless of what the oil can suspend. My 617 engine reaches this level by ~5kmi so I can't really do extended drains there. However my two 603's were only 1% at 10kmi, so I could theoretically go to near 20kmi before changing the oil. I do it at 10kmi anyway, that's about once per year, which is fine with me. Here's a sample of my oil analysis reports - I have a newer one (240-250kmi) but haven't made it into a PDF file yet:

http://www.meimann.com/docs/mercedes/oil_analysis_87b.pdf