If I am not mistaken, I think synthetic is the recommended oil, even for the 6cyl.

Just check your manual, as long as you are running one of their recommended oils and viscosities, its all good.

Its more critical in the turbo car because the oil gets hotter, and can break down faster.

 

The best thing to do is toget an engine flush and after that addfully synthetic oil back on your car. There wont be any damage from the old oil. Fully synthetic oil has less wax in the mix, therefore at higher temperature the oil wont get so thin and it will offer more protection. I would recommend you to follow factory specificationsspecially Audis.