Question about valve guide seals

I'm slowly rebuilding my 351W heads and only found 8 teflon positive stop valve seals. I believe I did not loose any because everything else is complete (locks, retainers, etc). Its been years since I took these heads apart.

Under some applications are these seals installed only on the intake valves? Or exhaust valves?

Which seal is preferred today, Teflon or Viton?

Here is the ones I have now-

I just received my viton valve seals (16 total in the kit). If this motor is basically a street motor should I use all 16? Or since the heads originally used seals on the intakes, should I stick with that?

Also, I have another new set of heads (for another motor) ready to go and I noticed manufacture used some "white grease" in the guides. Does anyone know what type of grease this is? Could I just use moly lube in place of this white grease on my 351W?