5.9 valve stem seals

Need to replace some valve seals on my 5.9, it currently has the positive stop PTFE style seals but I see mopar also has Viton umbrella style for these motors as well. Magnums have an 8mm stem diameter (approx. .315") and I have not been able to find the PTFE style seals with that size on the internet. The motor has used oil since it was built so I'm wondering if the seals in it are the wrong size. Does anyone have a preference towards either umbrella or the PTFE style? Does anyone recommend a particular seal they have had success with?
Any advice is much appreciated.