Once you've replaced those valve seats with hardened ones, you don't have to worry about adding lead substitute in every fill up. You do still have to worry about getting the right oil additives though, as the old flat tappet engines need more zinc than modern oil blends have in them. Ethanol in the gas is also a concern, but any conscientious classic car owner has hopefully replaced all the rubber and plastic bits in the fuel system to ethanol safe ones.
I think (don't really know) alloy steel valves and hardened seat were pioneered by the aircraft industry and appeared in cars by the 1950's. So really lead wasn't needed to extend the life of valves after that. Also my experience with some shitty 1960's cars was the valve guides would wear out before the seats. Compression is fine but engine burns oil and fouls the plugs. You could see it with older cars when going downhill, they'd be blowing oil smoke.
