Again: Unleaded or leaded fuel for mid 70's /6?

RustyRatRod said

AV gas is ok on the outside. The octane is great. It has other additives though, pertaining to dealing with high altitude that automotive engines do not benefit from. People brag about running it and making power, which I'm sure it can, but it's not the best way to go about it.

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As Charlie pointed out, the RVP is one of the big differences. When it is low, it prevents or minimizes vapor lock, which is why "low RVP" Summer fuel was mandated for high temp regions. One of the few govt. mandates that worked in carbureted gearhead's favor. But when You take that fuel up 5-10,000 feet, it's going to boil like crazy at the slightest provocation. RVP is very low, it has to be. Winter fuel is already in the pumps, close to 2 months now I imagine, it would be the highest RVP blend to ensure fast cold starts.