When you remove the Turbo and the Manifolds look inside the Intake and Exhaust ports and see if either Exhaust or the Intake ports have Oil in them. That will give you an idea where the Oil is coming from and going to.

I have more Blow-by than I would like as do quite a few others. The consensus is that if the Blow-by is not having a poor effect performance or anything else just drive the Car. (If you have enough Blow-by or a restriction in the Vent system it will shut your Engine down or has been know to blow the front Oil Seal out or push the Dip Stick out.)

If your Compression Test comes out well with in a normal range and they different pressures of the Cylinders does not vary too much I see no need to do a leak Back Test.

If Your compression test is close to being too low or the Cylinder pressures are too unbalanced then you might want to do a leak back test and hope the low compression issue is being caused by the Valves.

Few people do it this way but the Compression test is supposed to be done with the Engine at operating temp. That is what the specs in the book are geared to.

I am pretty sure if you dump Marvel Mystery Oil into the Cylinder and Crank the Engine with the Compression Gauge on you would end up igniting the Marvel Mystery Oil and the Cylinder Pressure is going to ruin your Compression Tester.
There is a tiny chance that could happen with Engine Oil.

If you just want to see where the leaks are you can simply use your compression Tester adapter hooked to Filtered Water free compressed Air to pressurize the Cylinder and listen to see if you hear Air leaking past the Valves or into the Crankcase.

If you go to the Harbor Freight site and click on the Leak Back Tester and look close you will see that you can access the PDF Instructions. Note that the Leak Back Tester is setup for Gasoline Engines and you would need to find a way to hook it to your Compression Tester adapter.

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84 300D, 82 Volvo 244Gl Diesel