.... I use shell v-power gas, as well. The owner's manual says it needs 91 octane. I wish I lived somewhere that alcohol free gas was easy to get.
Your reading the Owner's Manual wrong the Concours was built for and will just fine on 87 octane; there are two methods for obtaining octane readings... RON and ____ (I'm drawing a blank here) ....
Min 91 Research/87 Avg. Oct. Unleaded OK
Unless your live in another country the latter method is utilized for at the pump published octane reaings....
BTW I deal with the corn squeeze in the gas by adding Stabil, Marvel Mystery Oil, Startron etc to the fuel on a regular basis.