Glow Engines use a mixture of oil and Fuel to power and cool the engine. Mixtures are highly critical to the performance and longevity of your engine.
     Castor oil is the longtime mainstay component of the glow fuel.  Castor is not completely burned in the cylinder during combustion.  The unburned portion of the oil absorbs heat from the engine and is exited through the exhaust port of the engine therefore carrying heat away from the engine.  
     Unfortunately it does leave behind residue in the engine that eventually cause plug failure due to fouling.
     To resolve this issue engineers have developed synthetic oils that burn cleaner.  The very fact that they do burn more completely in the cylinder makes these lubricants less capable of cooling the engine.  A proper balance of castor and synthetics can provide balance that can provide sufficient cooling yet leave little residue behind in the cylinder after combustion.
     It is not recommended to use a mixture with less than 5% castor in most glow engines. 

CAUTION
Follow the factory recommendations for the proper mixture for your particular engine.