[RoundelM3]

First of all, all modern BMWs (except the diesels) require premium (also known as high-test) gasoline of at least 91 and preferably 93 octane due to their relatively high compression ratios. This goes for normally-aspirated as well as the turbo engines. Secondly, "watered-down" doesn't make sense for RaceTrac or any other gasoline. It's either 91- or 93-octane or it isn't. If they're selling 87- or 89-Octane as "Premium", that's blatantly against the law and your state government would nail them for it. What distinguishes one brand of Premium-grade gasoline from another is the additives, such as ethanol, detergents or other similar chemicals. Look at it this way - there aren't specific refineries for every brand of gasoline out there, so unless the station's tanks were contaminated with water, there wouldn't be much difference in RaceTrac's high-octane and any other Premium fuel aside from ethanol content.