I just got a new-to-me 1974 Cig 20 (details on that later) and on the valve cover it reads:

WARNING - HIGH COMPRESSION ENGINE
USE AVIATION 100/130 OR RACING FUEL ONLY

As best I can tell per the casting number (#3970014), this engine is original to the boat circa 1974, standard 350, 2-bolt main.

The previous owner indicated that after a long storage period they had drained the tank an put in fresh "racing fuel". Honestly, I didn't believe him at the time as he is really not a boater, but now I'm re-thinking.

What fuel should I be using? Will I need to use an lead additive?

wouldn't be a lead thing cause av-gas has always been low lead, hence the name LL100.

Low lead 100 has more lead than leaded race fuels.

+1 100LL is still loaded with lead. the old green 130 had up to 4gm per gal

Interesting sticker though. Avgas is very different from raceing fuel, and usually the benefits over premium pump gas are negligable.

So.... what should I use? In my opinion this is NOT a high performance engine by today's standards. Hell it's 37 years old!

I did test compression and got 110 to 120 psi across all cylinders.

With today's fuels would you suggest a premium pump gas (like 91 or 93 octane) or just use standard 87 octane?

Fuel additive or not?

If it had any kind of big compression that dictated high octane race gas I would expect to see 160+ psi and that would be with a stupid big cam bleeding off compression, more like 200 psi. If yours has 120 I would be inclined to run 87, was probably jetted in the old days of non-ethanol gas though so don't be surprised if you need to jet it up a little with 10% ethanol, Smitty

And it smells good too!