G'day You don't say what engine you have so it is not possible to say what is the right plug for you.

The heat range of plugs (hot, medium and cold) describes the way the plug affects the timing of the fuel ignition. A hot plug will cause the fuel to ignite earlier than a cold plug (ie, it advances the timing of the ignition). If you use a plug that is too hot, you risk getting pre-ignition - ie, the fuel ignites too early and may result in damage to the engine. If you use a plug that is too cold, the fuel will ignite too late and you may loose power.

You can also affect ignition timing by adding/removing nitromethane, changing the compression ratio and by adjusting the needle rich (retarded) or lean (advanced) but that is another very long story.

Most 40 size two strokes work well with the OS #8. This does not make it "the best", it just works well in most applications and it is widely available. I recently tried to run a Leo 46 with 10% nitro fuel and an A3 OS plug. The result was a disaster as it could be clearly heard pre-igniting in the mid range (a sort of crackling noise). I fitted an OS #8 and the result was much better but not perfect so I ran less nitro (5%) and the problem was fixed. It also ran happily and very economically on straight fuel with no nitromethane.

Four strokes are a different kettle of fish. They need a fairly warm plug that will retain its glow through the exhaust cycle. The OS F and other four stroke plugs are designed to do this. They often work well in some two stroke engines.

Older so called " loop scavenged" engines with baffled pistons seem to need the older "idle bar" plugs. The bar is there to try to stop large drops of fuel from quenching the plug.