My 1985 CA 300D has a different frame-mounted air cleaner. M-B simply tee'd off the valve cover vent, with one tube going straight into the turbo inlet duct and another tube going to the air filter housing (downstream of air filter). The upper oil pan has no oil drain tube stub. Perhaps the plan was for oil mist to get sucked/dripped into the turbo inlet, but the oil filter housing would also get oily at the bottom, at least when I bought the car w/ 279K miles and lower compression (guess ~280 psig).

After the OE engine failed at 330K, I installed a used engine (1982?), keeping its "federal" intake manifold and exhaust. Since the turbo sits lower, I cut the air filter housing outlet tube 1.5" shorter to match and used a new silicone hose elbow. Off the valve cover vent, I tee'd a tube to that engine's oil drain tube (where air filter sat), which seemed smarter than running the oil gunk into the turbo. I've noticed no oil at the air filter now, though that engine has perfect compression (>400 psig all cylinders). I also get no oil gunk on the turbo inlet vanes now, so even though I have no official "oil separator", the oil mist seems to settle out on the walls and run down that drain tube fine. I liked that approach so much that I fit a 1985 CA frame-mount air filter to my 1984 300D (has OE oil drain), mainly because it kept breaking the air filter mounts and bracket. I don't know if one could retrofit an oil drain stub to a 1985 CA upper oil pan. I understand there is a small check valve inside those, though don't know if mandatory for it to work.

A few photos which somewhat show it in both cars. Those who fuss that the 1985 CA air filter is "not avail" and/or very expensive never thought to stack 2 cheaper air filters as shown.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans