Thank you for my oversight.
Yes, viton.
However, there are not any threads between the fuel rail inlet and the outlet on the fpr. Simply an o-ring groove on the male end that fits into the bore on the fuel rail. And I would not want to try and seal that with teflon in place of the o-ring. Also, should any of that teflon tape break loose and slip into the fuel stream going into the fuel rail, that could be costly. Alos, I did not see any o-ring on the bottom of the fpr between it and the hard fuel line going into it. Just a standard SAE flared tube end going up into the bottom of the fpr, which generally does not use a o-ring, but I thank you for the mention of that, and I will check with my mechanic tomorrow when I pick up the o-ring.
SAE standard flared tube ends genearlly seal without the aid of any teflon tape or an o-ring.
Also, isn't the fuel line going into the bottom of the fpr the fuel delivery line? If not, oops, where is the delivery line?