I have had the best luck with a Viton seal with a steel jacket. Everything that touches the valve and guide is Viton, but there is a steel sleeve around it. When I tried some seals that had a Viton seal to the valve but a steel sleeve to the guide(smaller OD) they leaked like it didn't even have valve seals. I have had the scenario with two different cylinder heads. The Viton with steel jacket has worked both times and the other style has failed.