If this has not been covered I offer the following: 1) Our BMWs contain "High Performance" engines which means our cars have a high compression ratio compared to regular engines. 2) The higher the octane, the more stable the fuel. (please feel free to correct me on any point) Effect: Lower octane fuel ignites under less compression than higher octane fuel. Our high compression engines will prematurely ignite lower octane fuel before the spark does (the sound of knocking). Our engine computers will then compensate for the premature fuel ignition by slightly altering the timing (of the spark I guess). Doing so will prevent knocking however your engine is now operating at less than peak performance AND you have introduced a timing variable that is adding additional stress to the engine (as in igniting the chamber before the cam for that cylinder reaches the end of it's stroke which I). The stress may or may not have any long term effect engine wear (I simply do not know). Also, you will receive a drop in MPG (to what extent varies due to several environmental factors). My advice to all is not to purchase a vehicle that you cannot afford to operate and maintain. Life happens to us all so one may have purchased a BMW and then life through them a financial curve ball. Beyond that exception, if the difference in cost between octanes in gasoline factors into one's monthly budget I question the rationale behind purchasing this vehicle vs a lower cost vehicle with far better fuel economy. Showroom floor today would price our cars at $60,000 to $80,000 (if the e39 were still being manufactured today). Trying to drive an executive class car on the ultra-cheap is an embarrassment. Don't be mistaken, I've made several bonehead decisions throughout my life (will make more I'm sure), purchasing a car I can't afford just happens to not have been one of them.