My situation is different from yours in that my Clipper has a high-compression V-8 that definitely prefers 93 octane gas. When I was kid pumping gas in Southern California, premium gasoline was 97 octane. I do not know what the premium octane rating was in 1955 when my car was built, but I am sure that it was higher than the current 93 octane.
Gasoline additives increase octane, as well as horsepower.
My reason for using an additive in my car has nothing to do with the fact that I use unleaded gas -- my issue is my car's sensitivity to ethanol, which noticeably affects my engine's performance. Ethanol is the enemy of antique car engines. (See
http://www.fuel-testers.com/list_e10_engine_damage.html "Gas-caused (E10) Engine Damage and Performance Issues").
The bottom line is that our cars use carburetors, as opposed to fuel-injection systems specifically designed to accommodate 10% ethanol, which is the current federal standard. Ethanol, which is a solvent which absorbs water and dilutes the efficiency of gasoline, specifically:
* E10 reformulated gasoline, 111,036 BTU per gallon
* Non-ethanol gasoline, 114,000 BTU per gallon
* Diesel and kerosene jet fuel, about 129,000 BTU per gallon.
Ethanol will also corrode metal and rubber engine parts and gum up your carburetor from the release of accumulated deposits in the engine from ethanol's solvent properties.
The bottom line: Our Packard engines were not designed to accommodate a gasoline/ethanol blend of fuel. That is the key reason I use an additive designed to counter the effect of ethanol in the fuel. I use Sta-Bil 360 when I fill up, and Sea Foam about once a quarter. There are plenty of other good additives, but I have never used Marvel Mystery Oil in my fuel system so I don't know whether it is good or bad; but if it combats the effects of ethanol, it can't hurt.
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