Originally Posted by OmaHahn
I don't understand what you're trying to say. 93 octane is 93 octane, end of story, and ethanol actually has higher octane than gasoline. The only real issue with ethanol is that it has lower energy content than gasoline, so a 10% mix of ethanol will give you about a 3% decrease in gas mileage. But there's no issue with the octane rating.
To the OP's original question - as long as you use fuel rated to the car manufacturer's minimum octane recommendation (which is 91 of Audi in the US) you will optimize performance. There is no gain in buying 93 octane over 91. One other point - Audi recommends the use of gas brands that meet "Top Tier" guidelines - see: http://www.toptier