Ghostbear is right on.

I've noticed that the OS FX really prefers the OS plugs, especiallyt he #8. (OS "long" plugs are slightly shorter than other "long" plugs, which may be part of the issue with the .46FX.

However, other engines may or may not like the OS plugs. Some experimenting is in order. Some variables that affect plug choice include the amount of nitro in your fuel, the compression of the engine, and your expected RPMs.

Plugs are rated based on their "temperature". Hotter plugs cause the fuel/air mixture to ignite sooner or ignition occurs with less compression. It's kind of a mix between adjusting the spark timing and the fuel octane in your car, if you know anything about that kind of stuff. If your plug is too hot, you get pre-ignition. If it's too cold you get a lousy idle (and some other problems). For some unknown reason, most plug manufacturers will not actually tell you what temp rating their plugs are, you have to dig around for the info.

The good news is that most engines will work pretty well with most medium temp long plugs in a typical sport flying setup (factory muffler, 5-15% nitro, no engine mods, etc).

FWIW, I use K&B 1L plugs in my engines. It's a pretty good general plug, and it works in my OS .25FX engines (and all my other 2-strokes). However one of my students tried one in his .46FX, and it didn't work well, so we put an OS #8 in and the engine ran better.