If you are buying a premium car, at a premium price, and you want the premium warranty that is included, I think you need premium fuel.... 91 and above is perfectly fine.
And always try to have proof of this, since BMW likes to nit pick to get out of paying (they always have an excuse) for warranty issues. So either get a receipt or use your CC or DB.
BMW and Mercedes Build Beta Products!
Lexus build Release Candidate Products!
Why does this matter?
CAUSE EVERY TIME someone says Lexus has better build quality, one should consider what Toyota their Lexus Engine and chassis platform comes out of and how long that part has been tested in the real world!