[Oldsguy]

When I rebuilt the C heads on my 455 I added added hardened valve seat inserts on the exhaust valve locations before grinding them down to match the new valves because of what you are asking about. I am told it is true that since these heads were manufactured before the common use of unleaded fuel the best bet is to do that to prolong valve seat life. I understand that the heads built after unleaded fuel was commonly used have hardened areas around the exhaust valve seats. So, if you have original C heads or any other of the older ones for that matter, then it is safest to use a lead lubricant additive or suppliment. Although, I have also read that the newer fuels have lots of additives in them now for that purpose. I think I would rather be safe than sorry though.