It's pretty simple. Octane is a combustion inhibitor. The higher the octane the less volatile the gas is. That is why in high performance engines that run at high temperatures and more importantly high compression ratios, high octane is needed to keep it from combusting too early. It does not make more power or clean your engine or anything else. In fact you want to ideally run the minimum amount of octane that allows you engine to run w/o "knocking", or having the fuel detonate before the piston reaches top dead center.
SO, a B12 with 9:1 compression ratio can run on regular unless it's really hot and under heavy load. I say that because I could get mine to knock, so I ran 89 octane and it was perfect. There was never a need or point in running premium.
How "hot" your sparkplug is can also play into it somewhat.
You didn't mention if your Audi has a 1.8T, but that's what's in my Passat and I always run 93 in it. Although Turbo engines almost always run much lower compression ratios as tested, the effective compression ratio goes way up once you come on boost. Thus it needs premium. You can run 89 or even 87 because the knock sensors will retard the timing enough to let you get away with it, you will lose power and MPG to the point where it doesn't make it worth it financially. Also in the case of VW/Audi and most other high end brands, any engine damage that results from lower octane fuel will void your warranty. They can test the fuel and also read the fault-code off the VAG-COM when they do the diagnostic.
