Glow fuel
Glow engines should be run slightly "rich."  This means that the mix of fuel and air going into the engine has to have a bit more fuel in it than it can burn at once.  This unburned fuel and oil help keep the engine cool.  After a model has been run, it is coated with a film of oil and must be cleaned.  Ordinary window cleaner and a paper towel do a nice job of removing the oil.