I just went out and looked at the jug of fuel I am using , it is made by Morgan and it says OMEGA 15% Four Cycle Synthetic on the sticker .. I have no idea what the difference is but I have always used it in my four strokes .. I run mostly OS ..I never seem to have any problems . I use four stroke plugs in the four strokes..I always keep a few spares but seldom ever need them myself ..I did use up a few when I had an on board glow driver .