The 81 300SD Crankcase Ventilation System is a Valve Cover with some baffles in it and a hole in the Valve cover with tubing that I think in your case goes directly into the Air Inlet before the Turbo.

On My 84 W123 the Crankcase Vent Tube is routed into the Air Filter Housing throuh a Breather that is supposed to seperate the Oil from the Vapor and the Vapor goes into the Air inlet.

So as long as all the that is open there is nothing to the Crankcase Vent System.

On My year and Model is not uncommon for these Engines to have enough Blowby that the Breather/Oil Seperator in the Air Filter Housing cannot do the job and Oil can get on the Air Filter Element.
And mine was doing that exact thing.

If soaking the Cylinders in something will not help the Blowby; and I tried that on mine and it did not work you just have to live with the Blowby till such time as you wish to go through the Expense to overhaul the Engine.

Blowby can also come from worn out Valve Stems and Guids in the Cylinder Head and I think also from worn out Valve Stem Seals. I changed the Valve Stem Seals on mine and if it helped I could not tell.

How much is too much Blowby.
If your Engine Seals are reasonably good and you get enough Blowby the Crankcase Pressure will act on your Vacuum Shut off and push it into the Shutoff position messing with your idle.
Some People have had the Crankcase Pressure increase their Oil leaks.

I am not recommending that you do this but at hot idle if I block off the Crankcase Vent in the Valve cover in about 7 seconds My Engine starts to stumble on the way to shutting off.

If your Oil Fill Cap is not dancing around when it sits there loose your Blowbay is not at all bad.

Please note that someone testing blocked off their Crancase Vent and did that too long and the Front Crankshaft Seal popped out.

I have the Factory Service Manual CD Set and the actual Books and I have not ever seen a Crankcase Pressure Test in the Manual. But, I admit I have not specifrically looked for that.

Stretch has the German Version of that Manual and claims there is whole bunch of stuff in that Manual that is not in the USA Version. So you might PM him and ask.

Just for kicks.
If I remember correctly on other Engines you connect a Water Manometer to someplace that is exposed to the Crankcase Pressure and use that to measure it with. A Manometer is a U shaped Tube with long legs on the U and on each side there is an inch or metric ruler. Water is poured inside of the Tube and when you start off the Water is level on got sides.
The measuerment would be in inches of Water

You connect one side to the pressure and that pushes the Colum of Water up the other side and and you use that side to measure. The Measuerment would be in inches of Mecury.

To check Exhaust Back Pressure the U Tube would be filled withe Mecury.

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84 300D, 82 Volvo 244Gl Diesel