You have my problem.
My air filter gets oiled by blow-by.
The car runs great, uses little oil, yet I change the filter every 1500 miles.
One way to see is open the valve cover oil filler while she's running, if smoke puffs out, you've got blowby.
Quick way to tell how sound the motor, I'm told, if there is smoke it is an object of concern. (possibly low compression or valves)
That being said, I didn't know this when I bought the car, and I love my car, and it runs good. So I'm kinda glad I didn't look for that when I bought her.

What do the real MBZ mechanics think?

'80 300CD,
non-turbo, but GOES!

Long live the MBZ diesels!