Originally Posted by The Griz
There's a sticker on the gas cap that says "minimum 89 AKI (Anti-Knock Index). That settles it. Medium grade. Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), and (R+M)/2 are all the same measurement.
The sticker says "Premium"; minimum intermediate 89. To me that says use premium; if not available it's OK to use 89.
IMO I don't need to pinch my pennies - if I did I would not be riding a new BMW - so I buy the better grade - and I buy it at the better gas stations.
But, I also don't worry if I have to buy a lower grade because that's all that's available - I just take it easy on the throttle til I fill up again with premium.