I just got my AC going while doing a bunch of engine and other work. It hadn't worked since at least when it was parked in 2006. I flushed the A6 compressor really well with an entire bottle of PAG 150, flushed the condenser and evaporator with AC flush and compressed air, replaced the orifice tube with a new blue one, and replaced the lines in the engine bay and the filter/dryer. I wish I had also replaced the condenser with a newer parallel flow type, but it works fine as-is. I also replaced all the O-rings with the newer green R134 compliant ones. Then I vacuumed the system for a few hours and checked for leaks before filling with R134. No issues since.
Important:
If you decide to use the OE system and for anybody out there considering retrofit to 134a. Use an Ester Oil not PAG, PAG oil is a corrosive and will take out the evaporator.
FYI
This is incorrect. PAG is insoluble with R12 and the mineral oil that was used with it. PAG will also have a chemical reaction with water to create an acid that will eat through the metal components in an AC system. That's why you have a filter/dryer and need to vacuum an R134 system for at least 40 minutes before charging the system. It makes all the moisture in the system boil off.
If you can't get the system fully cleaned of the mineral oil(or just don't want to bother with it) then you can/should use the esther oil as it is compatible with both R12 and R134 and will mix with the mineral oil fine. If you are fully and properly switching to R134 by fully cleaning/flushing the system, then use the proper weight PAG for the compressor you are using. Esther oil only comes in one weight and might not be correct for your compressor.