I was wondering what the differance was when mixing the gas? The guy I bought this Boat from was useing Pennsoil synthetic and left me 1/2 a gallon. Should I, or can I change back to regular oil? Someone told me to use some high end premium oil in this old '68 100hp evinrude.<br /> Also I was thinking of runnig some Carbon Free thru the engine. When I changed the plugs they were kind black and gritty with carbon. Any thoughts? It ran great for about 20 min.s, then started to sputter almost like it was missing on a cylinder. Any advice would helpful
