Running 87 AKI doesn't do any damage. Neither does running 89 AKI.

You MAY lose power and mileage. You may not. It depends entirely on your local fuel. Fuel varies so much across North America in terms of quality that it is impossible to take another reference for your local conditions, unless that driver is using the same pumps you are.

My best mileage ever on my X5 was with 89. No loss of power.

Only way to know if the timing is being retarded is to try a full tank or two, and measure your mileage carefully. If it doesn't lose any mileage, go ahead and keep using it. If you notice a difference, decide if the penalty is worth it.

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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