Most snake oils simply take your money, but a few have actually worked for me. I wonder if anyone ever noticed a difference from adding STP to a gas car in the many decades it has been sold. Some say that Chevron's Techron actually works and isn't just alcohol (used to be Techralin, or vice-versa).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans