Well, I've never heard him actually say that. He has said you might have a slight difference in emissions which is why they tell you to use Premium in the first place. He has said if you drive your race car 160MPH and switch to premium you might get 166MPH out of it.

Actually on Sunday night/Monday morning a caller specifically asked him about running regular in a BMW and rather than me paraphrase it anyone that wants to hear can download it here:

www.drbill.org

grab OpenLine2006-11-12.mp3

Its 30MB (fast download speed though) because its a 3 hour show, but the BMW fuel question comes up at precisely 2:47:58 so you don't need to listen to it all.