In looking more closely at the "oil" and scum on the radiator cap, I think they are from the water wetter I added last year as it is clear not chocolate.
The various water wetters and "ices" all seem to be some combination of soaps and Surfactants. The soap is how they reduce surface tension to cool hot spots. And seem to clog stuff up in the bargain
Now that you mention it, this is the other thing I noticed the one time I used Water Wetter. Made the antifreeze look "dirty" and contaminated...and oily.
Regardless of how many people on the old interweb seem to like and promote it, the product flat-out does not impress me and I will never use it again...
