Really?! I'm surprised that you need to ask this question. The higher octane fuel you put in your car, the better the engine will perform.
You'll hear people say that octane ratings are a scam, but this is simply not the case. These people are in denial (& tend to think that octane ratings are a oil company scam).
I don't want to write a book (and this subject is somewhat complicated and has many variables), so here is primer:
I'm assuming that you don't understand what an octane rating is, so a simple explanation is that the octane rating of a fuel describes the fuel's ability to resist detonation. Detonation is the fuel/air mixture beginning to burn
before the spark plug ignites the mixture. This is caused primarily by a combination of high heat and pressure. That's why forced air induction is greatly enhanced by an intercooler - the intercooler reduces the temperature of the charge air and allows greater advancement of the engine's timing before detonation occurs (as well as increasing the amount of oxygen available - cooler = more molecules in the same volume). Pressure is related to compression ratio (& the actual amount of the air fuel charge. Forced induction = more air/fuel).
The higher the octane, the more resistant the fuel is to detonation and consequently, the long the engine can wait to ignite the fuel on it's own (called "advancing the timing"). The 3.2L motor has a compression ratio of 12.5:1 (which is pretty high for a production car). The Engine Management Unit of the car will advance the timing (which increases available torque) of the ignition until detonation is detected at which point timing will be retarded a little. Low octane gas will cause detonation to occur sooner so the EMU will be forced to retard the engine far more than is optimum to prevent knocking (which if allowed, will quickly burn holes through the tops of the pistons).
A couple of examples:
My '99 Mazda Miata is supercharged (w/ an intercooler). The temperature of the air charge at the intake manifold reaches a maximum temperature of about 140 F at full boost. I can safely run the car with the timing advanced 11 degrees without fear of detonation with 92 octane fuel. Experimentation has shown that detonation begins to occur at about 13 degrees of timing. I have never run lower octane fuel through the car because the car is not equipped with an anti-knock sensor and under boost, the motor could literally be destroyed in seconds.
The dragster I crew on is normally aspirated and we run the ignition advanced 20-22 degrees with 125 octane racing fuel (you couldn't really call the stuff gasoline).
The bottom line for you and your Audi is, you can (probably) safely run lower octane fuel in your car. Your performance and gas mileage
will suffer.
Audi also strongly recommends that you run TopTier fuel in your car to prevent engine deposits. This essentially means that you need to avoid "bargain" gasoline (like Arco).
I hope that helps. I'll try to answer any other questions if you have them.
Of possible related interest. I took these pictures at Infineon Raceway earlier this year. Basically a self-serve Sunoco gas station. People were coming in and filling up their cars throughout the weekend. BMW's, Audi,s, and lots of muscle cars. (Apologies for the size if you're on a 56K connection, but really, isn't it time you got broadband?)