This is as you see an oil vent. It connects to a pipe shaped baffle inside the valve cover where a vacuam is drawn on the crankcase. See that it is in good shape and does not leak. The leaking could be aggravated in a high mileage worn engine.
The good news about a high mileage diesel is; the oil useage doesn't foul spark plugs or cause any other ill effects as long as you keep oil in it. You can still drive the car as long as you can carry enough oil with you to get where you're going.
You should be able to get the necessary parts to fix it at the dealer. But I would first check with the Parts Shop on this website. If they can supply the pipe and rubber fittings, you will be money ahead.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in