From the 528i manual: "Because the engine-managment systems used in these cars incorporate predetonation-detection devices (knock sensors), you have the options of using fuels of different specifications.
Minimum requirements: Unleaded regular gasoloine (87 AKI).
Higher-octane fuels (premium unleaded, 90 AKI: super premium, 92 AKI) will provide slighly higher levels of fuel economy and performance."
Whenever I hear people talk about gases used and why, the "why" seems to differ. In the forum here, it is because of "clogged" fuel injectors (I agree with the above statement that it is poor gas or cheap gas not lower octane quality gas that will clog fuel injectors) that one should avoid 87 octane. Here in Vancouver at the dealership, by trusty salesman says to avoid 87 octane (availabe in Canada) and instead use 94 octane (available in Canada) so as to preserve one's catalytic converter for as long as possible if one is planning on keeping the car for a long time.
What I do is buy 94 when the gas is real cheap and when it climbs high put in 92. So in reality whats in my tank is a combo of the two and I let my "sensor" figure it out... lol.