From many previous posts here it seems the car will adjust to lower octane but you won't get the performance you paid for with the BMW.
Maybe more important is to stay with a top tier brand .... http://www.toptiergas.com
From what I've been told by my dealer the worst thing you could do is use a gas with excessive ethanol content.
"sometimes you're the windshield .... sometimes you're the bug"
2014 328i Mojave Metallic
2009 328i Black Sapphire Metallic
2007 328i Black Sapphire Metallic
2007 Honda S2000 Berlina Black
(Hey Jim... am I diversified?)