I have seen good results with SOME oil additives over 30 years of playing arround with cars and stuff....but I have also seen some pretty ordinary additives that where, either never good or have been overtaken by the general state of technology or better additives.
I have seen low friction plastics (teflon and the like) products work very well in engine oil in older motors.
I am very suspicious of any additive that is added to all oils in all applications.
Changing the friction properties in gearbox oil for example can cause issues with the syncros, that work on friction.
Likewise if you have a limited slip diff...that also works on friction.
One particular product known for its "copper lead molecules", was recemmended in any and all oils.....hell there have been some things gummed up by that stuff.....and its a technology that has been overtaken decades ago.
Before the " teflon revolution" in the early 80's, there where lots of graphite and molibdim disulphide fricton reduction products on the market, I remember scraping half an inch of the stuff out of the bottom of the oil filter canister in my mums Morris 1100, after this stuff was used every oil change during the warranty period as specified by UK motors.
That technology too has been over taken.
There was a very good case for friction reduction and stabilising additives in oil and octain improvers in fuel in the 50s, 60s, 70s and right into the early 80s.
But the use of finely ground soft metals, graphites and the like was main streamed in standard specification engine oils in the 80's, and in a far more developed and measured form...and has probaly been overtaken....current good specification oils have a very strong detergent, friction reduction, stabilisation package standard....and that package consists of things we would have baught as an aftermarket additive in the past.
It would take quite a bit of, good evidence to convince me to use an oil additive these days...and that means detail about what it contains, how it works and independednt tests.
If its ya engine you are interested in....here comes the cracked record again....the best thing you can do is put a modern correctly specified oil in the sump.
Unless you have a very late model car you cant go past one of the top spec, mixed fleet, diesel spec oils like, Shell rimular, Mobil devlac, Caltex dello or Castrol RX super https://mechanicguides.com/best-oil-additive/
ANd ya wont need an oil flushing chemical either.....get where I am comming from