FrogTwat,
i started running this mix last year after i bought my first 4s, i used to run straight 20/5/75 mix of full castor but my 4s didnt like it, after talking about & reading everything i could on fuels i settled on this mix, i run OS#3 plugs in all the 2s and OS4s plug in my 4s..
So now i have 1 fuel for all engines, 2 sorts of plugs and a bonus of a lot less mess than straight castor