What's happening to my (oily) skin?

[Claudine ME Jones]

Noticed today that while working on separate programs that require spacebar, the key got stuck, creating untold grief. Getting it 'unstuck' required taking the skin off. It is functioning as usual now, (since I'm typing this) but the byproduct of this temporary glitch is that it was immediately evident that many of the central keys: fghj enter bn spacebar, are all covered in an oily substance that is on the *underside* of the skin.
Before I freak out, can someone explain this? Is the rubber degrading? It's got to be at least 5-6 years since I got this and probably another 5-6 years since I've been using a Typematrix keyboard. Never had this issue before.

[Geobert Quach]

I have this oily thing too for years now, and I still don't know what it is. I think it our skin natural oil that goes through but it's just supposition.