Last week I noticed that my radiator fill tank had a gooey brownish substance (almost like watered down oil) in the clear fill tank. So I completely drained the fluid, thinking possibly the previous owner had mixed fluids, and replaced with new. When I actually drained the radiator itself, it was nice and green. It seems only the overflow tank is getting this way? Truck runs perfect, nice and cool, ect... WHAT THE???? HELP
1999 V10 140K

 

Has it come back to the reservoir?

 

Sorry I didnt clarify that. Yes I drove about an hour and now have the same gooey stuff in overflow again??