Technical Viton or PTFE Valve Stem Seals?

I'm ordering valve stem seals for my SBC. I see seals made from Viton and some from PTFE. Is there an application where you would use one but not the other. What is the application for each? I have been using Viton, but I couldn't give you a reason why. Just curious what the difference is!
I guess I am particularly concerned with the heat of the exhaust valve.
Any insight?

 

I currently use only Viton seal material VTO small OD seal. They are metal to metal at the valve guides. All my guides are cut to .500” to be the same on all engines, stockers or racing. I cut them myself with a Comp Cams tool. I purchase them from Competition Products.
Years ago I used the Teflon but changed for some un remembered reason. I’m sure they will work just as well. I still have them in my Bonneville travel box.

 

I used those small OD valve seals on my .531" guides. They are a bitch to get off. I had to get destructive to remove them. My dual springs don't require them so I'm going with the larger OD seals with the Viton all the way down this time. They are much easier to remove. Do you have a trick for removing the metal clad seals easily?

 

I wedge them off with a fairly large screw diver. I chuck them anyway. My reason: After 2 races with a new set of Dart Cast Iron heads assembled by a shop with excellent parts I removed the rocker covers and found red plastic appearing pieces. 1/2 of the seals were inside the springs and we removed the pan and found red pieces on the oil pump pick up. Luckily I had the tooling to remove everything on the head still on the engine. It is a single spring head because we run a flat tappet cam by the track rules at #140 psi. I felt the spring movement did damage but the smaller diameter metal to metal was a better choice.

 

If you're forced to burn ethanol laced gas, AERA recommends sticking with .002 stem to guide clearance & just o-ring seals if the engine will not be in constant use.

 

How bout some different seal pics? I'm ready.

 

If there is room I ran the umbrella seals like a ford uses that does not grip the guide .

Otherwise I was still running the Teflon seals as I have a bunch form years ago .

I like a little lube on a stockish street engine so I like the umbrella or just the factory oring in the retainer like chevy did for the last 65 years or so .

 

Viton seals are more forgiving if you're a little heavy handed on installation. Teflon seals work well but can crack or chip easily.