Slick50 + Splitfire plugs. Unfortunately, I also fell for both.
LOL me too! And DuraLube, STP, etc. Like slick 50, the old DuraLube (early 90's) did make the engine run quieter. This may or may not affect anything other than sound, but a quiet engine makes me happy! I don't know how much it would help with modern syn oils.
Oh, almost forgot the splitfires...tried them on OPE and a car, but never had anything positive to report. Throttle response was worse than OEM.