Many years ago all of our oil changes included a can of STP because Richard Petty said so. Never seemed to hurt a thing, and we were adding it to 10w40 back then.
I have used it more recently out of experimental curiosity in my Jeep, and again no issues whatsoever. It has fewer additives than the oil you are adding it to, but it does have some (unlike the Lucas garbage that costs 4 times as much). If anything cold starts were a bit quicker with STP in the sump, it is essentially not much more than a heavy dose of VII's (viscosity index improvers).
STP can still be relevant today in an oil burner, or if you find yourself wanting to thicken the oil a bit during an OCI. Otherwise if doing an OCI it makes much more sense to simply pick a thicker oil. In most sumps adding a bottle of STP will jump your oil grade up one grade (like 20 to 30).