Any experience or opinion on the professional engine flush (such as Bilstien) that some dealers/shops offer? I'm getting a used A4 and am not sure if the previous owner used only synthetic.

 

Check out www.bobistheoilguy.com for info about ARX. It's an excellent product that includes a clean and rinse phase to clean out the gunk. Go to Additives and do a search - lots of info. I have used this product in 2 cars, including our A4 with great results. I also use another product called Lube Control for long term cleaning and to help prevent oil oxidation.
I'm not tied in with these products and I don't believe in snake oils, but the info on the site along with real world experience has made a believer out of me.
I think that you can do the flush yourself for way less $$ than a dealer will charge, plus there is no issue of washing any bearing surfaces down or corroding them with a strong solvent. ARX works over an oil change interval.
You can also get some good info on recommended oils/filter for the 1.8T's.

 

I've always heard that a flush is not recommended on a turbo car due to how fine the bearings are and you wouldn't want any broken down sludge running through there, but that is only only what i've heard, I've only ever flushed the n/a cars that i've owned.