No, you probably should not run a fuel additive.
Lead
The exhaust valve gets superheated by the exhaust flowing through it. With an unhardened exhaust valve and seat, once the valve closes, it and the seat can micro-weld to each other. Then when the valve opens next, the metal pulls apart like taffy. This roughens the meeting surfaces, and they become quite abrasive. The pounding/turning of a valve with such "pulled" metal on it creates a nice grinding wheel effect on the seat, grinding it down so the valve sinks lower and lower. This is valve seat recession. In addition, the roughened surfaces no longer seal against each other properly, which eventually allows still-burning combustion gases to flow through the "closed" valve, causing a blowtorch effect on the poor valve and depriving it of any prayer of a chance to cool while it's on the seat. The blowtorch effect rapidly deteriorates the seal further, snowballing the seat recession. Lead halides act as a buffer (a physical "cushion" barrier) to prevent this happening.
But this bad stuff happens only if the valve/seat junction reaches the critical temperature. Whether that happens depends on how the car is driven and used. Towing, drag racing or pedal-on-the-floor hauling will heat up the valves a lot hotter than driving down the highway at a constant 70 or hopping from traffic light to traffic light in the city or running down to the local grocery for a carton of ice cream. Additional factors in the margin of safety include the size of the exhaust valve, its material, and the efficiency of valve seat cooling in that particular engine design.
Very little lead is required to prevent the localised welding and taffy-pull-apart effect that leads to the abrasive surface which, through incidental or positive rotation of the valve, eventually grinds down an unhardened seat. The majority of the lead was in the fuel as an octane booster, that's all. It was widely used because it was a very cheap and very effective octane booster. When unleaded fuels were first widely introduced, there was generally only one grade of unleaded available, and the octane was less than that of leaded regular.
We all know that when you use a fuel of insufficient octane, your engine pings (detonation, pinking, pinging, spark knock--call it what you will.) This creates tremendous heat in the combustion chamber--certainly enough heat to push the exhaust valves to the crucial temperature. Because for quite a while only unleaded fuel of subregular octane was available, plenty of people experienced these effects from using unleaded. While many of those engines that suffered under this low-octane unleaded really did need the lead (high load and/or high-RPM engines), the bulk of the failures were due to the low octane increasing combustion chamber temperatures.
These days we have wide availability of high-octane unleaded fuels -- remember, the pre-1972 octane numbers in the USA were higher for any given level of knock-resistance because in '72 the rating system changed.
if you have an old car that is a low-stress application, used in
daily-driver service, then you need have no qualms about using whichever octane grade of lead-free fuel your car runs well on and drive it for a long time with nary a valve or seat problem. Most slant-6 engines fall into this category.
The way to eliminate even the possibility of valve heating causing localised welding and subsequent seat recession is to install hardened exhaust valve seat inserts and exhaust valves of upgraded material. This is utterly standard practice in the rebuilding of cylinder heads, and has been for years. It is foolhardy to rebuild a "leaded" cylinder head without these upgrades, which are not expensive. The factory's induction hardening is frequently not sufficiently deep to last through more than one regrind.
Of course, there's no reason to tear into the engine solely to install hard seats. There is no collateral damage from seat recession. Drive and enjoy until a problem develops, then address the problem.
"Lead substitutes" are largely useless. Some of them use a sodium salt and claim to duplicate the buffer effect of lead, but can't substantiate the claim. Some of them use MMT (methylcyclopentadienyl manganese tricarbonyl), which is highly toxic, is of highly questionable benefit in buffering exhaust valves. It also causes hard red deposits on your spark plugs that will cause you to need to replace them more often. Other additives are simply octane boosters of varying minimal effectiveness and varying side effects (except on your wallet -- they cost a fortune!)
Ethanol
Many (and ever more) of us don't have a choice to run straight gasoline any more; in more and more areas you can only buy gasoline that contains ethanol (or as many carburetor men call it, "deathanol"). It's not corrosive (that's methanol you're thinking of), but it does create problems by dint of carrying water through the system so parts that never saw much of any water on straight gasoline now are exposed to water fairly constantly. As others have mentioned, the big aggravator is sitting; this steeps the carburetor and other fuel system components in a brew that contains water, and corrosion is the result. This is why you see white "metal mould" corrosion when you open up a carburetor that's been running (and sitting) on gasoline that contains ethanol. There are plenty of additives marketed as preventing the damage. Does any of them work? Don't know. Sta-Bil makes one such additive and I use it when I have to leave my carbureted cars sitting for extended periods of time. I don't think there is an additive worth using on a continuous basis in an attempt to counteract the ethanol.
Ethanol is also deleterious to various soft parts -- various plastic and rubber materials. This is a pretty easy to manage issue: replace the fuel hose with fuel injection hose marked "30R9", which is proof against whatever flows through it and has much less permeability and much higher burst strength. It's expensive by the foot, but most cars don't need much of it -- a few short lengths in the engine compartment and one short length back at the fuel tank. Be sure and use the correct Fuel Injection hose clamps, which have rounded-off edges that won't dig or cut the hose. And use a fuel filter with a metal can, not plastic. I like the new-design carburetor inlet needle and seat supplied by www.daytonaparts.com (they come in their carb kits and they're available as individual parts, too).
Gasoline with ethanol causes and aggravates issues with cold starting, hot starting, and driveability. Slant-6ers can do the Fuel line mod; everyone can add a carburetor heat shield. Everyone who hasn't already got a vapour-bleed provision in the fuel line between pump and carb can also add a vapour-bleed fuel filter (NAPA Gold 3040, for example) and run the ¼" vapour line (proper fuel line hose and/or metal pipe, please, not just generic rubber tubing) all the way back to the rear of the car. It's usually a hassle to add the vent nipple at the back of the car. One of the easier ways is to remove the fuel gauge sender and add the vent fitting to its round plate. If you are working on a '70 California or '71 50-state/Canada car, you needn't go through the hassle and (manageable) danger of attaching a new tube to the fuel tank or sender, you can add the vapour line fitting to the top of the vapour separator in the trunk of the car; see here for detail on how the '71 (+'70 California) evaporative emission control system works. While you are reading up on it, take note that it will aggravate the hot start problems ethanol fuel creates, because fuel vapours accumulate in the crankcase after engine shutdown and flow freely into the intake tract (making the mixture too rich) as soon as you sit down and start cranking. You can eliminate this part of the problem by moving vapour storage from the crankcase to a '72+ type charcoal canister with a purge valve as described here.
Ethanol contains less energy than gasoline, so a gallon (or litre) of gasoline with ethanol contains less energy than the same amount of gasoline without. That means you get less work out of any given amount of fuel, which means reduced fuel economy and reduced maximum power.
Time was, ethanol was considered a contaminant in gasoline. Now that the ethanol industry has bought the best lobbyists money can buy, we're all supposed to learn to say "ethanol enhanced" gasoline instead.
