Tom:
Nobody has mentioned it here yet and I am only doing it because of my experience.

I had an oily sheen on top of the coolant in my radiator. I also had the foam you mentioned at the top of the dipstick tube. Had that one time, due to a bad head gasket. The last time it happened, it wasn't a cheap fix like a head gasket. Turned out that my block had developed a fine crack.

Had to get a replacement L-head. Now that the block got rained on a while, you can really see the crack since rust has highlighted it!
Hope your situation is minor, like some of the ones mentioned here!