Isn't it also the metallurgy or some of the older engines? I know my 83 505 I didn't need lead supplement as the valve seats were hardened and we're happy with unleaded but other older engines needed the lead as lubricant.
The funny thing is that a '69 engine ran unleaded fine, but a '72 allegedly didn't.
In the end, I ran the '72 on unleaded and waited till the valve seats started to receed and then I'd strip the head and then machine the head for larger valves as possible.
Isn't it also the metallurgy or some of the older engines? I know my 83 505 I didn't need lead supplement as the valve seats were hardened and we're happy with unleaded but other older engines needed the lead as lubricant.