That is definitely oil fouling.
The valve guide seals are the cheapest alternative, so go ahead and do them, they die from old age. Chances are they are rock hard and useless at the current moment.
If that doesn't fix the oil consumption, you will need to look elsewhere -- you can check the valve to guide fit while changing the seals, probably -- no perceptible motion sideways. If you can rock the valve stem back and forth sideways and feel it, the guides are worn.
Synthetic oil leaves considerably less residue, by the way, another reason to change over. And you do need to get the oil problem fixed, the ash is abrasive.....
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!