My 2008 535i owner's manual (Canada) says minimum 87 AKI, and some other words about recommended AKI. They appear to be more relaxed about Canadian fuels. Your problem is that you get too much crap fuel in the US, so BMW has always recommended higher AKI as an insurance policy to allow for the fact that some of your 91 AKI isn't really 91 AKI despite being so labelled. They have been doing it for years, and have many owners in the US convinced that their BMW engines can only run on 91 AKI.

91 RON is pretty close to 87 AKI for many fuels.

I do agree with Fraser that the newer engines have better DME controls over knock.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue