Your BMW 325iS was built as a "sports coupe" and as a result it was entirely engineered for performance, from the suspension to the engine. You need to choose the correct gasoline, not only for performance, but also to maintain the health of your engine.
Now the easy answer is to tell you to use "premium unleaded gasoline" but, not all premium gasolines are created equal. To get the true answer you must dig further. When evaluating a premium gasoline you must look at the yellow "Minimum Octane Rating" sticker featured on the pump. This minimum octane rating is referred to as the AKI or Anti Knock Index by BMW in their technical literature.
Choose a premium unleaded fuel with a minimum octane rating of 90 AKI.
