When the valve stem oil seals are working well - which they generally do! - the sealing face of the inlet valve gets very little lubrication other than from the oil mist in the crankcase ventilation fumes.

Therefore, I would recommend that you should check your crankcase breather system, and make sure it's all free, and that the oil mist can get to the back of the inlet valves.

These engines, particularly the rearmost 2 cylinders suffer from inlet valve seat wear and recession - partly in the case of the NA OM606 by the fact that the routing of the crankcase breather pipes is a bit tortuous for the rear cylinders, and partly as the rearmost cylinders tend to run a bit hotter than those closer to the water pump and radiator.

I found serious inlet valve seat recession on my engine, which was causing compression loss, and it cost me about £600 to fix (I had new nozzles fitted to my injectors while they were olut, so, in total, about £800).

What I would suggest you look at is the EGR - if you can find a way to disable that without tripping your EML, then, it's worth trying - the combination of EGR and crankcase fumes tends to form potentially restrictive deposits inside the manifold.

For a bit more background on valve seat recession, see - http://www.tribology.group.shef.ac.uk/research/research_projects_valverecession.html