I have learned that oem's do a lot more testing than us owners and while they may be conservative to avoid warranty issues, they generally can be trusted. Most cars say Premium recommended or required. My Chrysler Crossfire SRT6 says Premium Fuel Only, which tells me it is required. That is what I use. The A6 just says Premium Fuel, 91 octane. So it is unclear whether it is recommended or required. When I was driving 600-1000 miles a week, I tried to save a little money by using midrange without noticeable effect. Now that I drive it much less, I alternate half tanks of midrange and premium. She has 216k on her and although the cats went at ~200, she is running good so I must be doing something right. You can do what you want -- caveat emptor. Good luck.

 

When I bought my 2012 3.0 the dealer not only said to use 91+ gas but they also went over a list of Recommended gas dealers (stations). I do not agree with that list however.
for the most part I use Sunoco when I can or other major gas manufactures like Exxon or Shell. Sometimes I will put in 91 oct gas with No Ethanol to clean things out.

 

Well, each year could be different. Read the owners' manual. My 2000 A6 2.7T manual overtly states that it is designed to run on regular, although premium is preferred. My 2001 BMW Z3 with a 2.5 6 states exactly the same recommendation and runs just fine on regular.