Octane levels in gas relate to the compression ratio of your engine. The higher the octane level the higher the compression level the fuel is able to withstand. Higher octane allows the fuel to remain a gaseous state and not combust on its own. BMW's run higher compression ratios in their engines which require the higher octane levels. By running lower octane fuel your fuel would be prone to combust before the spark ignites it causing the knocking sound mentioned above. Without going into great detail it's like slamming doors shut in your house. You can do it a few times but eventually stuff starts to break...
Premium is required but u can use mid grade to avoid future issues
I've always put supreme on BMWs. You may get some knocking or poor performance with regular.
Yes you can but it is advised you put Supreme.
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