I haven't tried the F in an Evolution engine, but I have used Saito SS 4 stroke plugs. I think the SS plugs are somewhere between the 8 and A3 from the way the engines react. I could be wrong, of course, but I would take the A3 over the SS. The whole idea is to get the Evo to burn off the fuel efficiently, which it does with hotter plugs. All of the Evos that I've been around stumble, sputter a bit, and have throttle up problems with OS 8s.

I'm not an expert on glow plugs, but it seems the F keeps heat longer for 4 stroke operation. There could be a difference between getting hotter and keeping the element glowing. That's what I'm thinking right now, but I could be wrong on this too.

Your link to scooterworks lists the F as being medium hot.