My owners manual says minimum octane is 86. I've had two gl1800s and two Valkyries that I always ran on regular. The F6B has never seen anything but 87 octane except in New Mexico where I have a hard time putting the 86 octane they sell there in the bike. If the motor doesn't require 91 octane there is no benefit to putting it in the bike. If it doesn't require 91 octane and there is no benefit to running it I don't spend the extra money.