I just tried something that goes against prevailing views but here's what happened:

I always run premium (93 is only available here) as recommended by MINI and it runs fine. After reading about how ethanol increases the octane level in gas and knowing that ethanol is boosted in winter formulations and that ethanol may actually run better in higher compression engines I decided to try some mid grade winter formulated 89. My non-S MINI seems to actually run better than on premium. More responsive and better mileage. I don't know if this is due to the higher ethanol content or something else but it seems to work well. When I have tried mid-grade in the summer (summer formulated) it seems to hurt the performance. I have read recently that ethanol may actually allow higher compression vehicles to run better and not affect the mileage as adversely. There may be something to this ethanol stuff for higher performance engines.

 

The MCS and MC are formulated to ron on 91 octane...
here in NJ we have 89 and 93

the lower octane gas may reduce power by pinging and the engine retarding spark to keep it from pre-detonating. (or pinging)

The higher the octane, the MORE RESISTANCE there is to burning.
(the engine can run higher compression ratios on higher octane gas and make more power... the higher octane gas does NOT have more energy in it!)

The corn-alc. in E10 or winter blends is still 89 octane, same as the summer blends.

The ethanol WILL absorb more moisture in air and gas and THAT may cause issues with your car running well...
The ethanol will also cause issues with other seals in the fuel system, esp. in older cars... like my 1986 Porsche 944.