There was an engineering error. The bore for the guides was a tad too large. The guide (usually the exhaust) would drop down. The cure was to use a guide with a snap ring to keep it from dropping. There was not so much a problem of oil leaking around the guide as there was with the seal getting too hot because the guide no longer transferred enough heat to the head because the contact area was reduced when it dropped and the guide picked up more heat from being deeper in the exhaust stream.

Not responsible for advice not taken...