Well I own two vehicles; a 2001 Toyota Tacoma doublecab Limited with the v6, and a 2016 Lexus RX350 also with the v6. The RX takes full synthetic so I don't use additives in that one. But the Tacoma, I've been adding this stuff called "MOC Oil Conditioner" since I bought it new in '01. Toyota carried it back then but not any more. So I have been buying it wholesale right from MOC since then, by the dozen. Placebo...maybe. But I use it faithfully with every oil change of 3500 miles. Toyota changes my oil, I add the bottle myself afterward.
I too use 100% synthetic oil so I don't use additives.
I was always told that such products aren't necessary these days as long as you change your oil at regular intervals but may have been of some benefit years ago before the improved motor oils were widely available.
No oil additives here. With modern oils, most of that stuff is a gimmick and usually a waste of money. Besides, with the new variable value timing engines (VVT), and other designs, I wouldn't recommend ANY additives. New motors are specifically designed to run on a certain type and viscosity oil. That's partially due to small passageways that need to flow the proper amount of oil, and pressure.
Adding additives can change the characteristics of the oil.
I used to get GM Oil Supplement from the local stealership when I had a stuck or noisy lifter. It always cleared it up and the problem didn't come back for a long time. Haven't had that trouble in many years, I don't know if they even make it anymore. I don't use anything these days, just buy the best quality conventional oil I can find and don't skimp on change intervals.
Years ago we would put brake fluid in the petrol so it smelt like you were using Castrol R oil, never actually used Castrol R, as you had to remove all traces of mineral oil first, and it had to be changed each time bike was used, and super expensive, today with synthetic oils the era of Castrol R is gone.