I believe it oils from manufacturing tha materials. I notice that some Chienes skimmers (uncleaned) take 2 Weeks or more to break in. And most USA made skimmer 2 or 3 days or less.
I no 1 guy skimmer that took 6 weeks to break in. The reason we think is he ran it in his bathtub before he put it on the tank. his wife put oils in her baths and we figured the oils adhered to the skimmer.