Ive got oil accumulation in my '84 190e 2.3l 8v air intake box too. its coming from the valve cover conector tube that connects the valve cover, air intake box and some other tube that heads toward the back of the car. Ive got a KN filter on my car cause it was only $10 more than the fram paper filter and will last forever. The oil accumulation was getting on my air filter and ruining it. I went to walmart and pickes up a crankcase breather filter for about $1.30, its the one with the orange rubber housing that it about 3-4 inches long and about half as deep. I took the filter floss out of the rubber holder and cut it to fit in the small compartment in the airbox where the valve cover tube connects to the air box. The filter catches the oil mist/spray that comes through the tube but still allows the tube to breathe freely. make sure you cut the filter floss a little bit big so that it fits tightly in the compartment and doesnt get jostled out of place and that should be the end of the oil mess in the air cleaner box.
hope this helps