On a factory tuned X5, ie no change in compression ratio and naturally aspirated, nothing over about 91 AKI has any benefit. Remember that the anti-knock index (AKI) is the average of the motor octane number (MON) and research octane number (RON) methods, ie (M+R)/2. You can't directly compare it to the 98 RON in Europe. With anything over about 91 AKI, there is no more power for an X5, it doesn't run cooler, it is just throwing money away.

The qualifier to that is that in some regions of North America, higher octane fuels contain less (or no) ethanol. That doesn't make any difference to AKI or detonation, and thus power, but it does make a difference to your fuel mileage, since ethanol, in addition to making engines run like crap, has less energy content for a given volume (so you burn more). I sometimes buy 94 not for the AKI but because I want to avoid ethanol entirely, and we have a distributor that gives me that option. It depends on the price spread, and if the increased mileage is cheaper than the price premium for 94. Not all states/provinces permit gasolines with no ethanol, but mine does.

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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