Oil like substance in Coolant

[bigbodyla]

I own a 2002 impala 3.4l. I looked in my coolant container and noticed this stickey dark film all over the inisde of the container. It seems like oil or something. Does anyone have any idea what this is??

[tractorboy]

when the car is stone cold, take off the rad cap and have a look their also.
also take out your oil dip stick and see if there is mayonaise like stuff or the oil on the dipstick looks like chocolate milk, not to be confussed with just dirty oil.

I was also reminded by reading another post that it might also be stop leak that you are seeing. The factory put in stop leak to counteract a lower intake manifold gasket leak on some vehicles, mostly heard of the leak on Malibus.