Tom, I think it has more to do with the alcohol than the oil content because all three of my converted gassers have brown insulators. They all use the RimFire plug and the two bushing rod SuperTigre's get run on 16:1 mixed with either Redline two stroke racing oil or Pennzoil two cycle air cooled oil. I can also idle for extended periods without even the slightest hint of fouling. In fact the first plug in the 51 made over 35 hrs before I changed it.