I have used a 45R on my original 1960 Corvette for years and the car runs great. This is a hot plug and there is no problem with 91 octane California gasoline. R stands for "resistance" {sports car connector}. You can search the ACDelco spark plug chart and find the best spark plug for your region. I also recommend adding 5 ounces of Marvel Mystery Oil to the gas tank every other fill. The oil prevents fuel lines and carburetor gaskets from drying out due to the additives found in modern gasoline.
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