Audi's, like most modern cars, specify a minimum octane rating in their owner's manuals. This is typically 87, 89, or 91 octane (or RON equivalent in other countries). Using a *lower* octane fuel than recommended can lead to knocking, reduced performance, and potential engine damage. Using a *higher* octane fuel is generally fine and won't hurt the engine, though you won't necessarily see a performance gain unless the engine is tuned to take advantage of it.
To find the correct octane rating for your Audi, consult your owner's manual. The information is usually found in a section about fuel specifications or engine requirements.
