Originally Posted by got3fords
Yep, that's my opinion. I think this is being over thought. I have had more than one car that had crap build up in the overflow tank. Not one had a serious issue. I think some impurities got trapped in there, from Mfg assembly or something, and takes forever to burn off because its a sealed system. The tank is the highest point in the system, so it that's where it boils off to. Most likely, that's where it stays.
Seeing the same stuff on mine with around 6,000 miles and not worrying.
I noticed the same thing on my 2001 Silverado around the same time, similar layer of funk/slime in the top of the tank and on the cap. Suspect
it has to do with being a sealed system like on the focus. 246,000 miles
in Texas & Florida, never had an issue, and all I've ever done is flush it every 4-5 years.
Not saying some folks don't have an issue, but if it's only a light film/funk like in some of the pictures, it's probably not worth worrying about.