What you need to remember is most of these motors are manufactured in the US where their "standard" fuel is of a lower octane (88 from memory) hence our standard pump fuel at 91 is the equivalent of their premium however when I ran a two stroke I used to fill with 98 every tenth tank full as supposedly (and I am no mechanic) it assists in keeping the injectors nice and clean however I am sure the technically minded members will put you straight.    

suggested always running it on 95 octane. The few times when i havent been able to get 95 and had to run on 91 i noticed she was alot more smokey than usual and sometimes didnt want to start as easily so its always 95 octane for me when its available. 

The timing is set for the factory specifyed octane, higher octain fuel generaly requires more advance, or the motor will run retarded, hotter. Motors with a knock sensor will automaticly advance the timing. If you get the timing set for the fuel you want to use and stick with it, you will get a power gain,economy too , but need some one with the knowlege to do it. An intresting thought, outboard dyno.

my old johnson 2 stroke 97 ran noticibly better on premium unleaded, I premixed in the tank

my newer 4 stroke doesnt make a difference, some coastal hamlets have crappy fuel in their tanks probably rust and water ingress issues

 Thanks guys. My motor have been running fine on 91. Just thought using 95/98 will have more benefits such as cleaner running etc. But based on all of the above feedback, I'll stick to 91 and regular general maintenance.