the 250 engines are designed for 87. You'd have to alter the engine to be able to use higher grades without damaging the bike
Higher grade octanes burn slower. The slower burning can cause deposits in the combustion chamber/piston crowns. Higher grade doesn't mean it's better for the bike. People have had different experiences, but this is the most often occurrence. You'd think higher grades would give more power, but it doesn't
the bike is made for 87, and 87 is the cheapest!
Also, the 87 gives the best mileage for the bike. Again the false idea is that a higher grade would give more power, which it does not. All of that makes it pointless to use anything but 87
I've never put anything else in my gas. I think only the 11&12 are fuel injected and everything made before that, at least in the US, is not. So that's a no
As for adding anything else to the gas, you can always use a seafoam additive if you want. I've never used it, so I don't know the exact steps to take, but you're supposed to put some in there when you winterize the bike especially. I live in South Texas where I can ride pretty much all year round so I've never had to winterize my bike.
Use 87 octane ethanol free gas, if you can get it. The bike will run much better and you might squeak out a couple extra mpg, if the bike is tuned well and you ride conservatively. As far as additives go..just keep some fresh gas in it and you shouldn't have any problems. If it's going to sit for weeks then add a stabilizer. If you have a used bike, add a cleaner/ stabilzer treatment like Seafoam or Startron fuel treatment.
48-51 is about normal I think. That's what I'm getting around town, but on the highway I get in the upper 60's. I guess I ride aggressively, but the tuning may be a little messed up on my bike or something.