Years ago motor oils were somewhat primitive, and the use of additives to correct for engine wear or for "dirty" engines was sometimes necessary. Stuck rings, clogged lifters, scored cylinders, etc. were not all that unusual. It was not unusual for engines to be worn out and in need of a rebuild at 100,000 miles......cylinders with large ridges at the top of the bore was common.
Modern engines and motor oils are far different today. Fuel injection, better materials and better oil have all created an environment where a properly maintained engine can last a very long time. I just talked to a fellow yesterday with a Toyota Tercel that has 403,000 miles on it and runs fine. I just sold my Pontiac Vibe with 210,000 miles and it still runs flawlessly....... and I fully expect it to be one of those engines that can get to the 300,000+ miles if the new owners care for it.
While my wife is watching her shows on TV, I have watched several YouTube videos recently from Lake Speed Jr. - he is a motor oil "Tribologist"....he is a professional at engineering and and creating motor oils. His videos are independent and he is not sponsored by any oil company....or engine manufacturer.
In the attached videos he looks at engine additives. There are two things that he states in this video that pretty much summarize the very long and informative video.
1) If you need additives - you are using the wrong oil.
2) No OEM manufacturer recommends using additives in their engines.
He also explains that every motor oil you buy has a base oil that comes from one of a handful of manufacturers, and the same is true of the additives. Each company buys the base oils and additives and blend them to create their motor oil product. There can be as many as 14 additives blended into each oil - and when you add another additive you can create a situation where the ingredients combine and actually create an oil that is worse for your engine. As an example ZDDP at low levels can allow wear in your engine - while too much ZDDP is corrosive!
For this video the message is simple (but long). There is no reason to be adding anything to your motor oil - you are not going to be able to create anything superior to what is blended by the professionals.