Although I have a different engine than you do, my 300SD has a simliar tube. It runs from the top of the valve cover to the top of the air filter. Inside of the air filter I have a small oil accumulator, which as far as I can tell collects oil from the hose and dumps it back into the engine. Since my car was designed with an oil accumulator, I have deduced that it is normal for oil to go through the hose. I should note, however, that my engine has a small amount of blowby, so I get more oil in the hose than you probably do.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K