You have too much oil getting into the affected cylinders.
Could be valve guides worn -- 10,000km on semi-synthetic is probably too long, but still you shouldn't have that much trouble.
Do you drive short distances often? This can make the problem worse because you aren't burning off the oil completely -- it leaks down the valve guides when the engine is stopped.
I would also try some full synthetic oil and see what happens. It will probably burn off better and leave less residue, and may cause the valve stem seals to work better.
Otherwise, it sounds like a valve job.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!