<89 octane is fine for stock, I wouldn't recommend going any lower. I have a 325is with a chip and my son had the car for two weeks commuting and he ran 89 in it with the stipulation that he would not do any full throttle bursts, high revs, etc. and it was fine. I took it to an autox right afterwards, and since it had 1/2 tank of 89 in it I dumped three gallons of 115 octane racing gas in and it was fine.