If you look in the tower catalog they have a pretty good description of what plugs do what in different engines. OS #8 is the number one all around plug for two stroke engines. There F is the number one in use for four stroke engines. I have been trying out different brands of plugs in my four strokes due to the price OS is now getting for there F plug. So far I haven't noticed and difference. I'm waiting to see how long the different plugs last compared to the OS plug, that could be a deciding factor.