Krytox by Chemours GPL 205 Grease, Pure PFPE/PTFE, 2oz Tube

Q: How much does it take to treat the rubber seals(doors, hood,trunk,etc.)on a small car?

Min a third of this small tube ! Depends how good you can spread the grease, a heated garage if in a cold climate helps, clean all seals first, start in the middle of the seal so you can spread it over both sides ! Leave doors open for a couple of hours so all grease can penetrate in the rubber and not stick at the metal of doors, hood,... Remember this is a wonder product, expensive but more than worth it !

Q: Can this be used in place of plumbing silicone on the o-rings in a faucet?

It's a grease for lubricating o-rings, not a sealer. If you're looking at an alternative to silicone caulk, this is NOT what you want. If you're looking at an alternative to silicone grease for lubricating o-rings, this is much more expensive and there are more cost effective options