As far I was told OS is named one number highr than most other brands, so a RP6 is like a R5 and yes, that can be too hot.

You can ask the question if a colder plug will help but trying it for a few buch will give you the right information. A colder plug will shift the ignition and will result in a cooler engien, hom much I do not know, just give it a try.