spark plug heat range questions.

And the reason i run 91 octane fuel in this engine is because thats what it said to run in it with the instructions that came with this engine.
My old man bought this crate 360 magnum brand new in 2001 and i still have the paper work that came with the engine. It says we recomend using no less then 91 octane fuel. Compression ratio is 9:0.1.Although it isnt extremey high compession i have ben told that you can use premium fuel in an engine with that compression ratio, and mopar reccomends running the 91 octane fuel in it.