Phil, if you can get 98 octane I'd use it in your ZDZ. Going to a lower octane wouldn't gain you anything. If the engine will run fine on 91 it'll run fine on 98 too, but if you have to retard your timing on the 91 octane to avoid detonation then the 98 would actually help you make more power.

Dick, I agree with Wings that you'd get more power if you ran on 100LL and advanced the timing to 7mm.
Charlie