I think it’s a safe bet to say that Yamaha designed the S10 engine to operate at optimum performance while using 91 octane or above. That’s certainly not the same thing as saying “if you don’t use premium gas, your engine will suffer expensive damage”.
If you look at page 4-27 of the owner’s manual, you’ll see the fuel recommendations:
The manual recommends the use of premium unleaded gasoline, with a (US) octane rating of 91 or higher. That’s 95 octane in the rest of the world. Nowhere in the manual does it state that the mere usage use of a lower octane rating will result in engine damage. It does say that, if knocking or pinging occurs, to use a different brand of gasoline.
Notice that the manual is very clear as to what type of gasoline
not to use because they can or will damage the engine; leaded gasoline, and gasoline containing methanol. The manual is very specific that the use of leaded gasoline
will cause severe damage to internal parts, and that gasoline with methanol
can cause damage. If the manual delineates the specific use of one type of gasoline as dangerous to use, or even possibly dangerous to use, why would it also not be just as specific about a prohibition on any other type besides 91 octane or higher, if in fact anything but 91 or higher would or could damage the engine? The manual recommends that 91 octane or higher be used, but it does not say that damage will or can result if you don’t.
I definitely believe that there may be some performance compromises at using 87 octane gas, such as the ones mentioned by Checkswrecks and Tenereguy, and you might have to manage your expectations for performance with 87 octane depending on where you are. That’s not the same thing as “damage is imminent if you don’t use premium”. The Super Tenere, in my experience, isn’t sensitive to the use of a lower grade of gasoline the way my Harley was. Anything less than premium in my Harley resulted in overall poor performance and terrible knocking and pinging.
Eemsremo has over three times my mileage, but I’ve still put 103,000 miles on my S10, and many of those miles were with 87 octane in the tank, in all kinds of conditions, I’ve literally never heard the engine knock, and I’m familiar with what that sounds and feels like. My engine oil analysis at 88,000 miles showed less than average wear on the internals for a bike of that mileage. Nick Sanders’ experience on his Super Tenere during his world travels was even more severe, as many of the places he was riding didn’t have premium gas at all, and some of the gas was of pretty questionable quality. He had to do a lot of throttle moderation to keep the engine from knocking on some of the available grades, but even with thousands of miles on shit gas, his ultimate engine teardown showed a pristine motor.
I’m not advocating the use of 87 octane. You’re always on the safe side if your adhere to the manufacturer’s recommendations. And if you’re using 87 octane and your engine knocks, then don’t use it any more. If you’re crazy enough to keep riding it when it’s doing that with 87 octane, then yes, it’s very likely that your engine could get damaged, because that knocking is the sign that your octane content is too low. But if it isn’t knocking or pinging on 87 octane, then there isn’t some other secret damage happening to your engine that’ll result in it grenading unexpectedly at some future point in time. The whole point of the octane additive is to prevent autoignition, so if you aren’t experiencing autoignition, then whatever grade of fuel you’re using has sufficient octane for the demand you’re placing on the engine.