Can turbo bearings be cleaned in situ?
No they cannot. The turbo must be removed, disassembled and repaired as needed. There is really no “cleaning” of the bearings. If they are sludged up, they are destroyed. I’d also be concerned that the rest of the engine is also sludged up.
I’d suggest buying a rebuilt turbo and using your old one as a core. Make absolutely sure the oil feed and drain lines are clean and clear or the new turbo will be destroyed in short order.
What about a "Flushing" oil or other stuff? when changeing engine
Also, I would have thought that turbo-charged engines would have
their own special oil? not using regular engine oil.
I have used engine flush oil, once. I’ve put synthetic oil into and engine with high miles and Quaker State oil change stickers all over it. The synthetic stripped a TON of junk out of the engine. so much that I did my next change at 1500 miles and then again at 3000 miles just to get the crud out. Based on that, I would not use it today.
As far a “special” oil… Use the grade indicated by Audi. Probably 5W40 (that’s what mine uses). It will be on the oil cap and in the owners manual. 5W40 only comes in synthetic so there you are.
This engine is direct injected which means the fuel won’t wash the intake valves. Any crankcase gasses will crust up on the intake valve, especially as the cars get more miles. Consider adding an oil catch can system to prevent getting oil vapor to the intake and if you just got this used, consider a chemical cleaning of the intake system… or worse… intake valve carbon removal.
Also get ready to find out how expensive these cars are to service… $1200 for a turbo, gulp!