I can't talk about the Saito SS plug (though I'm sure it's a good one) but I have run OS-F's in all my Saito's. As one engine guru explained it (I think it's Clarence Lee, but not sure) the four-stroke plugs are designed to have the ability to retain their heat long enough to fire on every other stroke, typical of 4S operation (as opposed to 2S operation, which fire on every stroke).

Horizon Hobby used to recommend one of the McCoy 4S plugs (forget the designation) but perhaps this Saito plug is from that family.

Hope this helps somewhat,

Bob