I was running the 15% four stroke fuel (Omega) in my 4 stroker and the muffler quickly became a brown nasty mess. I was told from the guys at the field that this is from the castor content and going to a 2 stroke fuel would run cleaner, avoid the muffler problem and work fine as well. This advice came from an engine machinist who really knows his stuff about model engines. Either one appears to be ok but the only downside of the 4 stroke fuel seems to be the brown glaze from the burned up castor on the muffler. A wire brush will clean it up though.