If it were me, my first job would be to rinse the intercooler with petrol. Make certain there's no oil in it. Then rinse it well with water! (I know petrol and water don't really work, but it does flush out all but trace)
Run the vechical and then pull the intercooler again. Actually remove it again, as hot oil will run down to the base of it, without leaving so much behind.
Then you know for sure the zone. If it's oily, it's turbo, cyclone, or engine pressurisation.
If it's clean, then it's rings or stem seals, or possibly gasket
It just narrows it right down.
I had a 300tdi that just keept going smokey. I even replaced the turbo, and still it was dumping oil into the air stream. I took off the breather too, so it was certainly comming from the turbo.
Turned out to be a blocked oil drain hose from the cyclone (which I still don't understand, but that was it) causing the engine to pressurise and oil be forced through the turbo seals.