From Decalin Chemicals

"Some white deposits may be seen on the plugs and in the exhaust pipe during use; this is normal with RunUp since the lead in the fuel is being converted to non conductive lead phosphate. When no additive is used, lead oxide is formed. It is lead oxide which causes plugs to misfire due to its partial conductivity shorting out the plug. In addition, lead oxide tends to form in large clumps or clinkers whereas the lead phosphate is crumbly and if any is left behind in the combustion chamber it deposits in even layers."