It is hard to believe that Mercedes would ever implement this method that combines engine oil vapors & liquid with exhaust particles. On my car the thicker areas reminded me of asphalt. I used a small metal spoon to scrape the intake holes in the head. About 1/2 the intake holes just had dry exhaust particles.

I then followed this up with an attempt to clean the remaining residue via paper towels with a little bit of B12 spray onto the towel. I wrapped that around a 3/8" 6" extension and was careful not to go deep enough to contact the valves. Kept doing this until the paper towel wasn't picking up much more. Some valves have a good buildup of that asphalt coating but I don't know of a good method to clean them. Since fuel doesn't flow past them I guess they will stay that way for a long time.

I do not believe that the oil has any benefit for valves or valve seats. My guess for the valve seat problem is that cylinder has excessive heat from lack of proper cooling. From my reading that is the cylinder near the firewall/cowl that might have the problem occur. From what I've read the heat buildup in that area also makes the glow plug harder to remove. On my car that is the only glow plug where the harness wire connector was siezed to the glow plug. I had to peel it off with pliers, acted like is was welded but knew it wasn't.

I'm thinking about running a slightly lower mix of antifreeze and a little cooler thermostat in an attempt to provide better cooling for that area.