Do some searching. This have been talk about a dozen times.

If BMW recommends 91, then filled it with 91. Anything more is just waste of money. Unless there is only 93 available then 91 is okay. I have use 89 in the past on my E53 3.0 and E70 4.8 with no problem, because 91 or better wasn't available at the gas station. "89 is better than run out of gas" but I usually filled it up 92 or 93 because there isn't 91 available in Michigan.

Read more in www.toptiergas.com/

It is not important in term of octane in today modern Car. Unless it is a TURBO CHARGER, total different subject. Otherwise 87,89,91,92,93,94 even 100 will burn the same. It is the Detergent in Gas that really make the differents in the long run. Use what is recommends on the owner manual. If it ask for 91 on E70, then 91 it is. More is not better, just waste of money period!