I have been given advice to use top tier fuel in my Z3. Greatly appreciate this tip. I never noticed a problem with 87 octane until I was backing up a steep incline (heard pink; engine seemed to labor). Today I filled with 93 octane and feel the difference - or is it mind-over-matter? Also, I checked my mpg and seemed to be improving 1-2 miles per gallon. To some degree, this could off-set the extra fuel costs. Mobil gas was not listed on the toptiergas.com. I wish I had known there was recommended list out there. Oh well - I'll use one next time. Is Mobil a good fuel??? Thanks to all for communicating.