what gasoline octane rating is best for a 1980 C3 corvette?

Higher octane doesn't make the car go faster , 87 and 91 have the same potential energy , Most fuel has additives to run things cleanly . where I live the 91 has no ethanol which may be a good idea for an older original engine like the one in my truck , a 94 GMC ., it's only available at one convenience chain store , from what I know ethanol is an oxygenator and the one used in 91 is more expensive . High compression engines may ignite the lower octane fuel prematurely ( knock ) . Years ago guys would talk $h!t and say jet fuel from the airport would make the car go like hell because of the very high octane . But of course jet fuel is refined kerosene and is the last thing you want to put in your tank . People are idiots remember that , " an empty can makes the most noise " , " arrogance and ignorance go hand in hand "

Top tier 87 octane. However you may have carburetor, fuel pump and rubber fuel line problems with the E10 fuel of today. That’s what they are recommending high octane as some high octane fuels are ethanol free.

If this will be a daily driver you can use the 87 octane. But if the vehicle will be sitting for extended time periods between being driven I'd recommend using premium. As others have mentioned the lower octane fuels have ethanol in them while the premium doesn't or has a much less ethanol content. Ethanol absorbs moisture from the air and will cause the water to get in the gas tank and fuel system. As we know, water doesn't burn very well so over time, if the vehicle sits alot, you'll have problems. I have two vehicles that I don't use much and I always put premium in them. I last filled up my 2000 Grand Prix with premium December 31, 2018 and my 94 Fleetwood January 20, 2021. Without any additives both vehicles start and run just fine when I want to drive them. Hope that helps! Jim

If you do have the original owners manual that specifies 87 Octane, and you have the original engine, you probably have the LG4, 305. You should be fine with today's 87 Octane, as well. Don't be concerned about Ethanol. At this low percentage, it shouldn't cause any problems in your car.

You did say, L82 which is the low power 350. The same holds true for that engine.

True that the engines have no problem burning this E10 fuel the fuel delivery systems are another matter, if the vehicle has not been updated with ethanol resistant equipment it will suffer some issues. If it is going to run E10 fuel then it will need all the rubber fuel lines replaced with the new multi-fuel hoses, the fuel pump upgraded to ethanol resistant type, and the carburetor overhauled with the ethanol resistant kit. If this has not been done already.