I should have said "rubber like" materials, because Barnsnake is right.
Viton is a
synthetic based Fluoroelastomer developed by DuPont Dow Chemical co. I have used Viton based materials for other sealing/gaket applications and it is very cool stuff. There are 4 dedicated "types" of which certain compositions are not recommended for exposure to automotive or aviation fuels. Its ability to endure long term friction bearing exposure is what some builders question.
My builder has used both Viton and Teflon for valve seals. He has a bucket full of the Viton and other rubber type valve seals that he dumps off when they come in with cam kits etc. He swears by the longevity and durability of Teflon.