Ahhh, one I can answer from experience.
To achieve modern emissions laws engines have to recycle the crank case vapour, which in a diesel for various reasons is laden with oil. The process is assisted by a cyclone separator or a crude oil catch can system but still a large volume of oil is deposited in the intake stream.
Second point of introduction. The Turbo will always pass some oil, this too ends up inside the intercooler pipes and throttle body, inlet manifold swirl flaps in truth everywhere also.
The biggest culprit in this though is the CCV from the crankcase.
The solution is to give the oil somewhere to clout out on to. The worlds best system but sadly very expensive is the Mann Hummel Provent, more commonly fitted to large trucks to extend the useable life of the engines (hence over 1,000,000km routinely seen) it is like a remote oil filter with a replaceable paper media that you change at a suitable mileage. it traps 99.8% of all oil passing through it very effectively.
We fitted them to the Jeeps the 2.8 VM engine the 2.7 Merc 5cyl and 3.0 V6 and it made a world of difference. In fact I fitted one to a brand new Grand Cherokee that then got so much attention from the dealer involved they started to offer them as options!
The Mann Hummel provent is available via
https://www.mann-hummel.com/mhuk/upload/doc/HBUCDGJaind.pdf
There is another system but I cannot recall it as the Provent is expensive, however ask the question how much is a new inlet manifold or engine?
