Oil in air filter tubing??

[CJ]

I was doing some work on my 1991 300D 2.5 TD, when I had to pull of the rubber intake tubibg that goes from the airbox to the turbo. This tubing has a hard plastic tube, covered with rubber, that runs from the top of the valve cover to the airbox tube. I am puzzled because the inside of this tube was covered with oil and the tube from the valve cover does not contain any sort of valve in it. On my race cars I have tubes that goe from the valve covers to the airbox, but they have a PCV valve in them. What is the inside scoop on these? Is there supposed to be a valve in there? The only thing I could find in my research is that this particular tube is made intentionally of a hard plastic to prevent it from collapsing under pressure/vacuum. If it does collaps, then the diesel engine will leak oil like a siv.
Thanks for your help!!