Well, if the manual says that you have to use 91 octane, then yes, it's definitely true that the manufacturer wants you to use 91 octane, My manual says 91 or higher, or RON 95 or higher. In the US, Canada and Mexico it's 91 octane, in Europe, Australia and NZ (and probably everywhere else) its RON 95.
I can only speak from my own experience, but I don't always use 91 or 93 octane; my bike has seen many, many miles running on 87 octane. I've never had any incidents of knocking or pinging, they way I did on my Harley if I didn't use a higher octane fuel. My bike just rolled over 100,000 miles last week, and so far, my occasional use of 87 octane hasn't done any harm. My oil analysis done back around 89,000 miles showed less than average wear on engine parts. I live in a pretty hot climate myself here in the south, where 90+ degree days have been the norm since June (it's 97 degrees here right now).
Does your manual actually say that you will get engine knock if you don't use 91 octane, or that knocking may result if you don't use it? My manual says to use 91 or higher octane, and to switch to a different brand of gasoline if pinging occurs.
