Just bought an '07 A-6 with 5K miles on it. What grade of octane should I be using?

I was suppossed to use "super" on my '05 Infinity G35x but started using regualr when the gas prices took off and didn't see too much of a difference.

 

It should tell you on a decal on the inside of the gas door, or maybe a notation on the gas gauge (Premium fuel only, or something along those lines), or at worst, check the owner's manual. I'm not sure if you must run 91 octane, but you definitely do not want to run 87. Bong-water fuel and Audis don't mix well. For the record, my A4 calls out 91, which they don't sell here in NY, but I see virtually zero difference over a long period of time using 89 vs 93. I use 89 most of the time, and will use 93 maybe every 10th fill-up. No issues, no mpg problems, and the car isn't any slower than it ever was.