High octane is more resistant to detonation. Detonation can kill a two stoke really quick and you don't hear it knocking like a four stroke. However unless it's been modified you don't need more than the 89 octane Stihl calls for. With our blender pumps I get 5 gallons of 91 because there's a gallon or so of whatever the last person pumped before it gets to what I selected, and that was probably 87. This is probably over thinking it and a can of 89 would be fine.
My favorite oil is Redline Allsport. It makes the exhaust smell better than anything else I've tried and when I tear down the engines they look really good. I avoid Amsoil due to the skeezy MLM aspect. It might be fine but having seen MLM ruin family members I don't want to encourage a company that does it.
I agree with the keep it stock and run it crowd.