Those teflon seals are for race engines that need more oil on the guides. Go with the solid rubber Viton, the sets from Felpro work good also. I have seen a couple sets of the teflon type leak pretty bad. A good seal will hold a valve in place, but not so tight you need to force it to move. You should have to tug on them to get them off the stems, if not, they were to loose. I have the heads cut for the type I linked ( positive type ), it usually takes a deep socket to drive the seal on to the head, and the head and seal need to be dry so they " stick " to the iron along with the spring clamp pressure.
