I worked for Clark
Oil from 1974 through 1978. Clark wanted to sell Premium, leaded regular, and unleaded. However they only had pumps and tanks to carry two fuels. Since they were required to carry unleaded by law, they decided to make the unleaded a Premium 93 octane unleaded. They claimed it would not hurt the valves in the older cars. I was told that it was the 87 octane that caused more problems than the lack of lead, but I do not know for sure if their 93 octane unleaded had anything in it that is not in todays premium unleaded fuel (we may not be talking the same fuel - today verses then).
During those years I put about 24K miles on my 68 Dodge Charger using the 93 octane unleaded. Those were extreamly hard miles on a big block mopar. I never had any problems. I also ran it in a 327 chevy, a 350 chevy, and something else that I am forgetting. I never had any valve problems.
Although I would not suggest using the old seats for something I wanted to last 100K miles, I think they would last 20K. Trueth is the old seats didn't last much over 100K with leaded fuel either.