8 is the cooler plug and it is used on the bigger/more powerful OSs (basically FX/SX range) where you don't need the heat. The smaller/less powerful ones (basically LA range) have the hotter plug to ensure they run right.

You are I beleive right about the F being the hottest and why.

I have used an F on my 32 SX, but only because it was inverted and I couldn't get it to idle right. I used the F more because it is a longer plug, the idea being it gets it out of any pooling of oil. Didn't make a huge amount of difference though.