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Adding 100 Low Lead aviation gasoline will poison your catalytic converters and put you in violation of Federal Law. That would be an expensive octane boost.
If you want to add five gallons of something, add 5 gallons of toluene. It is a pure hydrocarbon (no oxygen, nitrogen, or anything else) with an octane rating of about 114. Gasoline already contains 10 to 30% toluene, so you won't hurt anything.
DO NOT ADD 5 GALLONS ADD 1-4 DEPENDING ON HOW MUCH OCTANE YOU NEED.