I have used STP Oil Treatment for over 50 years, off and on, in almost every one of my older vehicles.  I NEVER experienced any problems or issues.  I used it in both my Toy trucks every time I went to dunes for a little bit more protection because I was 4000 and 5000 RPMs most of time while running the dunes.

That's just my experience, everyone's experience may be different.

Gnarls.

Not harming the engine isn't the issue, it's if it's doing anything to help.  I'd bet you would have had the exact same experience without it.

This is how most people judge motor oil additives.  The label says it does "x" and I never had an issue running it so it must be doing what it says.

Lucas oil stabilizer is just pure petrolium which means is essentially a single grade conventional base oil without any additives which doesn't sound much like an additive.  It just thickening up your oil and "watering" it down.  Sure it often quiets things, but so would any 50wt motor oil.