If the engine is not consuming much base oil and starts and runs good. Basically all is usually well.

Some time ago I wondered why the crankcase developed more positive pressure with time as there seemed by examples like yours where everything else was well. My guess was it is old valve seals and some wear in the valve guides.

The intake valve guide clearances are not going to suck oil down them as the turbo charger keeps a positive pressure at the bottom of the valve guide. The intake manifold does not sit at a much of a negative or suction pressure even when the turbo is not providing pressure. At least compared to a gas engine.

Gas cars have a negative pressure there. That is why unlike many Mercedes gas cars changing the valve seals out gives little if any reward. The exhaust valve guide is subject to pressure at it's bottom basically as well. So one might see some exhaust gas component coming out of the filler hole as a result but not much..

One day I may think up a test but for now it really is not needed.

I am left to basically judge these engines by the current base oil consumption rate for expected remaining service in general. Plus how easily they start up cold in comparison to other known good examples if the fuel system and glow plugs are in decent shape.

Just the strange way I look at things.