ptfe, this stuff is used by plumbers, it repels water, not oil so is very little use anyway
Actually it repels both, it's fluorinated (PTFE = poly-tetrafluoroethylene aka teflon) so it's neither hydrophobic nor oleophobic. If it didn't repel oil all your non stick PTFE coated baking trays wouldn't work. Plumbers use it because it's thin enough to fill the gaps in the swagelok threads not because it specifically repels water.