You can use 89 octane, but you'll lose power. The ECU will detect detonation, aka pinging, and retard ignition timing. I don't know what the long term effects of using 89 in a B18C are, but why risk it? You can go back and forth between tanks of gas between 89 and 93 I suppose, since 91 is the recommended octane for a GSR, going back and forth between 89 and 93 will land you at 91. That how it is in N.H., gas stations have 87, 89 and 93 - no 91. Except for the Sunoco stations, but they're more expensive than everyone else. Why pay, for example, $2.07 a gallon for 91 at Sunoco when you can get 93 at Exxon for $2.10, you know?
Hey you, half the guys on this board would kill for a B18C! You're lucky enough to have one, treat it right!!!