You should be using 89 octane here in CO if your car recommends 91 Octane.
Octane loses 2 points in the transition from sea level to our 5200+ feet here in the Denver area.
So 87 octane in NYC is 85 octane here in CO.
89 Octane turns into 87 at high altitude.
91 octane becomes 89.
93 becomes 91.
BC.