I also have to go along with the CrankCase Pressure. Ideling ins one thing a long high speed trip is another.

I have also seen this on a gas car that my Father had. In his case the Valve Guides were worn out and if the Breather Cap on the Valve cover was left on Oil would come out from between his Oil Pan Gasket. I used to drive the Car with the Breather off and tears in my eyes due to the Exhaust getting past his exhaust Valve Stems.

So for some reason the Blow-by pressure at high speed may not be able go get out of your system fast enough.
I also wonder if some Camshaft or IP timing issue could also be making it worse.
Check out your plumbing and see if something is plugged.

If for some reason it turns out the Exhaust Valve Stem Seals on your car are shot at least they do not cost much. Less than $20. I bought mine but have not gotten around to changing them.

This is most likely not the problem but if your Air Filter is plugged up enough it can cause Oil to be pulled past the Turbo Charger Seal on the intake side. I have see this happen many times on the Big Rigs back in the 1970s.

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84 300D, 82 Volvo 244Gl Diesel