The best fuel (AKI rating) is the lowest one that will provide full performance.

As others have said, there is no advantage to running higher AKI ratings.

There may or may not be any negatives from running a lower than recommended AKI ratings. The sky won't fall.

That is because AKI is not a precise figure, and engines don't have a specific single requirement, but rather a range over which the engine is designed to operate.

Different driving conditions require different AKI ratings, ie summer and winter. Heavy throttle foot or light. Altitude makes a 2 point difference.

Actual fuel AKI rating also varies in practice. Fuels may be at the high end or low end at your supplier. There are wide regional differences in quality throughout North America.

Considering all of the above, BMW specs 91 because for most conditions in North America, that will be sufficient to never impact performance. Since all engines are different, and fuels range widely, that means that many are buying fuel better than they require. Fuel requirement (and supply) is distributed like a bell curve. 91 is at the far right of the curve; nearly everyone is to the left of it so nearly everyone will be fine on 91. But the most common requirement is something less than 91. Half will need less than that requirement, half will need more.

The test is to run a tank of 89, and see how it does. If you notice increased fuel consumption (measured over a full tank), or lower power, then that is a good indication that you would benefit from running 91. If you notice no difference, there is absolutely no reason to buy 91 over 89. The engine will run on 87, but it is more likely that you will notice a difference.

It is better to focus on how much ethanol is in the fuel, as that will have more impact than the AKI rating most of the time.

Best economy I ever got on my E53 was on 89, which I ran for four years. Same on the 325, which we ran for five or so years. The 535 noticed a difference, and ran best on 94. I used 91 or 94. The X3 runs better on 91 so I use that most of the time. When I travel, and don't know the local fuel quality, I always buy 91.

If all that seems like just too much trouble, just put 91 in it. But don't worry at all if you get a tank of 89 by accident or necessity.

Jeff

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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