That's the same thing my sales guy told me when the dealership delivered our brand new X5 about 10 years ago
"It's good to use 91+ all the time to get the maximum &optimum engine performance.
BUT you can also use 87 or 89 only when you have to". At that moment I lost my faith in him (sales dude)
** I always pay the extra .40c per gallon and use Shell or BP as much as possible