98 RON (Research Octane) as usually used in Europe = about 92 Pump Octane in North America (calculated as (R+M)/2, and also called AKI for anti-knock index) - and it is not a mathematical conversion as the test methodology is not the same for the research and motor methods of measuring resistance to pinging. It is usually about 5 or 6 points.

Using lower quality fuel will not result in pinging on the BMW engines as long as the DME can modify timing sufficiently. Taking it to extremes is obviously not recommended. I have never heard pinging on my 3 vehicles on my local 87 or 89, but performance and mileage both suffer on 87 due to the timing being retarded. I have absolutely no performance or fuel consumption degradation on 89, but that is my local 89. Fuel quality will vary widely with geography, which is why BMW specifies 91, to cover the areas that are on the low side. In other areas of North America 89 may definitely cause a performance degradation.

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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