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I always run premium in my M340i, and it performs well and gets great gas mileage. Just not worth the trouble to experiment with running regular unless that’s all that is available at the pump. I did try running 100% gas (no ethanol) a couple times when pumps offered it in areas with a lot of off-road vehicles. It was more expensive, but seemed to improve gas mileage.
It is also a little surprising that some folks in this forum will spend thousands on HP exhaust, expensive unlock fees for special tuning , bigger tire and wheels, and many other performance improvements, but then start wondering if it’s worth it to buy premium gas.
It also seems that running top tier fuel might be more important than octane, due to the direct injection BMW uses. Port injection injects fuel into the port before the valves, so the gas/air mixture scours and cleans the valves. BMW direct injection sends fuel directly into the combustion chamber, which is after the valves, so there is no equivalent cleaning/scouring of valves and valve seats. As always , YMMV.
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