If you put in less than the minimum octane level consistently, the check engine light may go on, indicating a fault code has been thrown up....could be P1384 - I've had this happen on my 328 and diagnosed it with by OBD II reader. You will only know this (and what it means) if you have a OBD II reader that can tell you the fault code and re-set it, otherwise the light may just stay on. And it may cost you a trip to the dealer to get it checked out and turned off - and that won't be cheap. Less than minimum octane will ultimately cause engine knock but could affect performance too. Not worth it.
2012 535xi M-Sport, Carbon Black, Cinnamon-Alum Hex, Exec Pack, Tech Pack, HUD, Damper Control, BMW Apps, Heated Rear Seats
Last edited by PeterC4; 01-22-2012 at 11:10 AM.