I've heard arguments from all sides, but what made the best sense is these Oil Mfgr's have done all the science and chemistry that we as Pikers just don't even have a clue to.
STP changes the entire chemistry and viscosity of what the Oil Co has designed and is a crap shoot to much with that. So me personally, I've gone Castrol Synthetics on everything I drive. On V-8's I'm getting 250-350k miles on my daily drivers. One went over the 350k mark. I sold it, and I saw it on the road some 2 years later.
Do as You wish, but realize these Oil Mfgr's are pulling out all stops for engine longevity...Wanna mess with the mix... Not Me!