I drained oil into a clear container and there was no water or brown sludge in it.

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