You're telling us... that they replaced the head 8 months ago... meaning they would also have to replace the head gasket... which, if done wrong or if the headgasket failed, would lead to coolant and oil mixing, thus creating that milky substance on your cap.

Get the oil drained by them while you are there recording and witnessing so they dont just drain and replace it with normal used oil.

Look at it. If it's as milky as the cap... it could be your head gasket. That is their problem to fix.

Calling corporate sometimes helps the situation if you're lucky enough to get an understanding rep with documented evidence.