I think that the suitability of your locally-available 89 AKI has more to do with the actual AKI of that fuel, than with your altitude. It may be fine, it may produce less power and mileage than locally-available 91 (which is the factory recommendation).

My local 89 was fine, no power reduction or fuel consumption increase compared to 91 AKI. Fuels vary widely across North America, and I don't know what fuels in your locale are like. Your dealer may be right, but I would check mileage with both fuels before assuming that.

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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