There is a carmal looking substace in my overflow tank

If there is no residue in the radiator that is good. Does the residue feel oily? Remove the coolant reservoir and wash it out as best you can use something that cuts grease (Dawn, Fairy Liquid any dish washing liquid cuts grease and oil). Refill reservoir and check in a few days. Oil will destroy hoses and make them soggy from the inside. Oil can only enter the cooling system, if someone accidentally "put" it there, if the head gasket is bad or the automatic transmission cooler leaks and allows fluid into the radiator.