I am going to expand on a few things in this thread. There is a good explanation already but a picture might help more. Not captured in the below picture is MON uses variable ignition timing to further stress the knock resistance of fuel.
95 RON is the same as (RON+MON)/2 87.
The old timer that talked about mixing fuel is talking about leaded gasoline. Way back in the day when you mixed low octane with high octane it would result in a number almost as high as the high octane version. It had to do with the chemistry of the fuel. That doesn't work any more and now you will just get the average.
Reading the plugs on a modern vehicle is not going to give you the information near like it did versus a vehicle which has a carb. The new vehicles have an O2 sensor (or several) which gives continuous feedback and adjustment of the fuel mixture. The old carbs are not self correcting like the fuel systems of today are.
Putting in 87 octane is not going to hurt your engine. If you do hear knocking or pinging then you need to step up an octane rating or find a different fuel. No knocking or pinging, your engine is going to run fine and cause no damage. If you find yourself someplace you can only put in 85 you can still make do with that fuel. Just don't lug or hot rod the bike, keep out of full throttle and drive it calmly and you will be fine until you can get to a place with the correct octane.
I do want to talk about the old timers giving advice. While they might have really known their stuff and maybe still do, the experience they have, which they are basing their knowledge on, might not have changed, while the vehicle technology did. The things my dad told me while he was driving his 1969 Roadrunner might be out of date when talking about my 2021 Lexus. Some things will hold true still and some won't be helpful and could even be detrimental. When the old person is giving you advice, just keep in the back of your mind how the changes in technology might have made the advice not as relevant today.
I will give a personal example. Long time ago if you had talked to me I would have told anyone wanting to put a turbo on a car that it is a bad idea. Performance, the lag, the headaches associated with them were not worth it (blow through carbs). I would have told you that they are for tractors and semi trucks, motors that run at a constant speed for long periods of time, and diesel was the best fuel to use with them. Fast forward to today and I would tell you turbos are great, the technology with the fuel injection has made them a very viable performance option. I had to change my thoughts because of the evolution of the industry. That is just one area, there are a ton of things I would have told you years ago that I wouldn't tell you today. Just like how you use computers through the years and the advice you would have given on the windows 3.1 system wouldn't apply to windows 10, same thing with vehicles.
fuel, and add additives, injector cleaners, an old timer that worked or owned a Shell station a long time ago said mix the high with the low 1/2 and 1/2 is best, I used to like Amoco but I don't have any around, they may be gone now?? In Florida they sell 103 octane at Sunoco, but it was like twice as much, tried it in my kx 250, didn't notice a big difference, seemed to run hotter, but I think my compression was down to 150psi. Ut I'm sure some I use BP cause it's close and the commercials say it's clean and has additives, I don't know what kind, but the site I posted pretty much says what Dr D said. Gl
