I really can't say why my STP experience was so bad, but after losing a pile of money on several rebuilds, I just got really gun shy about using it. I've been using 15W40 DELO oil for quite some time on engines with high mileage, or ones I think can use the extra zinc compounds. I'm not sure if they still add extra zinc to diesel rated oils, but they do perform well with the extreme pressure present in engines with 22:1 compression, which would be way above the cam lobe loading on a stovebolt. BTW, the letters DELO stand for Diesel Engine Lubricating Oil, something I learned back in the 1950's when it was reccomended for customers of Dad's shop whose engines were experiencing hydraulic lifter problems. Hydraulics gave a lot of problems, particularly the ones Olds and Cadillac used in their first-generation overhead valve V-8's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!