If your talking about the oil residue that gets on your airplane that you clean off before you go home, That is normal. Your engine wont spit out nice clear pink oil, like the color of gas you put in, Once it burns in the engine and spits out the muffler it just comes out as a darker burnt color. The only time you need to be concerned while running glow fuel is if your engine is not spitting out this oil residue, then you might be having some problems.