AKN 2.5L TDI: High Oil Consumption Problem

Hello everyone.
My A6, manufactured in 1999 and with 180,000 km, has a significant oil consumption problem. I need to add about 3 liters of oil when driving aggressively. The turbo is fine; I've already replaced it. However, I've noticed that the intake manifold is quite oily. I suspect it's either the engine ventilation system or the check valve. Yesterday, I removed the oil filter and noticed that the oil filter housing was about 3/4 full of oil, and it took about 45 minutes to drain completely.
Is it normal for it to take this long?
How do I access the butterfly valve? Does the pump need to be removed as well? What other potential causes could there be, and how can I further narrow down the problem?
I've also noticed a loss of power, a knocking sound during partial load, and when the engine is cold.
Following that, I replaced the turbo, and it worked well for about 500 km, but then the same problem occurred again.
Can anyone help me with this?
Perhaps I should add that, before replacing the turbo, I had the following error code:
Intake manifold control differential exceeded 575!
I am grateful for every tip.
Best regards.
Stefan.

Before you get shocked, have you already cleaned the engine ventilation system? The cover under the turbocharger and oil filter is held in place with two Torx screws and the ventilation hose. You can either replace the entire housing (which is available) or cut off the felt material and reinstall it. This is the most common cause of oil consumption and reduced performance. If the camshafts are worn, it won't necessarily require more oil.

It seems to be related to the engine ventilation system.
The vehicle is from the beginning of 1999, and it still had the original battery.
It runs much quieter now and doesn't make that knocking noise anymore. I'll have to monitor the oil consumption for now.
Have a lovely rest of the weekend and best regards.
Torsten.