We just flushed the coolant in my brothers 2000 7.3 155,000. We drained it, ran the hose in the bottle till it came out clear water and then blew the system out with the air hose. Refilled it with fresh premixed coolant and treatment. Now a week later a very thin layer of a oily lookin sh!t has settled on the top of the coolant in the bottle anyone know what that could be??! Will it hurt? Ne help or suggestions would be a big help.