One thing to remember is that both contact surfaces wear out, not just one or the other. So chances are that even though the guides may have been replaced later, the valve stems themselves had wear on them also, but weren't replaced as they should have been, decreasing the new life that was expected.
I'm afraid I can't answer the question about what guides were in the head and what the condition of the guides and valve stems were.
But now I see there are Viton seals that resemble Teflon seals and may be a good choice too.
I'll have to look into those.
Thanks.
