STP was an additive that goes back decades, when oils were not as good as they are today.
In my opinion, it is foolish to add additives to todays oils. They have lots of 'packages' added such as detergents, dispersants....& ZDDP & you risk upsetting the balance of those additives.
I think I read recently that modern oils are "saturated" with additives so if you try to add more, they don't dissolve and eventually end up separating out and collecting at the bottom of the sump.
On my older stock-cammed, stock-valvespring flat tappet engines I just run Valvoline synthetic with a quart of VR1. Duster currently has a 5.9L Mag short block with a roller cam and I run Valvoline 5W-40 "European Car Formula" in that. The previous 360 had a Lunati Voodoo HFT cam and I ran Penn Grade in that. For a hot rod-type car with flat-tappet cam that isn't driven every day the more expensive Penn Grade, Schaeffers etc is worth it to me but necessary? Probably not lol.
If you guys want to see some interesting oil tests check out Project Farm on YouTube. He refuses sponsors for any of his videos and has a pretty decent setup where he tests lubricity, pour point at different temps and other things.
While I have my preferences I have to agree with @318willrun , all motor oils these days are good and as long as it has ZDDP for your flat-tappet engine you'll be good. Also keep in mind many current OHC engines have flat followers/tappets but also spec certain oils for the same reasons our old pushrod V8s can get trashed from the wrong oil.