With 293,000 miles on my engine (87 300D Turbo), it was very dirty inside even though it had regular oil changes. Even though my compression is pretty good, we found some oil in the prechamber of #6 when replacing injectors. I've been smoking at startup and at idle.
My mechanic says that if we replace the valve seals without the replacing the guides, they won't last very long. Even if I could afford a valve job right now (which I can't) he is afraid that at this point in the engine's life with high mileage, I'd end up having to come back for rings and or more. Probably time for a complete rebuild.
In order to put off any major expenditures, we put a can of flush, (expensive stuff with a green label in German), and ran it for about 400 miles to try to loosen up whatever we could from the valves. Drained and changed the oil, I've changed it a couple of times.
With the regular usage of Redline DFC, my car seems to smoke less and run smoother. We put a plate between the EGR valve and the intake to reduce the oil.
I've done about all I can without major mechanical work for the time being. The only problem I've had since the oil flush which may or may not be related (I have a hunch the flush may have aggravated the situation) was that I had to replace the diaphragm in the injector pump so that the engine would shut off with the key.