I have heard the mid-grade recommendation in cold climates from BMW more than once.

Can't say for sure that this was what caused your misfire, but yes, high-octane fuel is harder to light up, and thus more prone to misfire, and a tank of 89 could make a difference.

Go back to the 91 when the weather warms up. No, the 89 can't hurt your vehicle. If you are experiencing any misfire, you will get better mileage on the 89, and the power difference won't be noticeable.

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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