That is exactly how I determined what the actual AKI requirement was for each of our vehicles, using multiple tanks. Only applies to my home base fuels, as different fuels will be encountered when travelling. Better than trusting the "lowest common denominator" recommendation from BMW marketing. But if it all seems like too much trouble, just use 91 and be safe.

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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