I don’t think running 90 octane is going to make any difference either.
For what it’s worth, 91 octane is recommended for my Lincolns and I’ve run 87 for going on 15 years and hundreds of thousands of miles with not even a hint of a problem. The mileage remains the same no matter the octane and actually, while visiting CO where 85 octane is available I run that. Oddly enough, the mileage in the mountains on 85 actually goes up about 1 to 1.5 MPG.