The only emissions change on my engine was addition of the AIR pump, which sapped about 10 hp. Unleaded gas was still mostly in the future. I know I used low lead in 1980, that was regular gas. In 1975, most cars had catalytic converters and that required use of unleaded, which cost more. TEL is a cheap was to boost octane.
Then ethanol happened, from corn. Ethanol is also an octane booster, a pretty good one, but it reduces the power inherent in the fuel. Premium gas can have a higher ethanol level than regular because of this, which means you'd get slightly lower fuel economy and power from premium.
40% of our corn crop goes to fuel ethanol, a thing nearly everyone agrees is nonsensical.