I met the late great Smokey Ynick in the late nineties and spent time listening to his great stories. He called the oil additive people snake oil salesmen. I asked him about STP and he told me at the Indy 500 one year the USAC official came by asking if he was using STP for the race. There is prize money for using a product and having the company name on your car. Smokey told the official he was about to put it in the oil tank. Smokey goes to his tool box, opens a can of STP, and gets an eyedropper out and puts in a couple of drops of STP in the tank and the USAC official Seals the oil tank. He told Smokey, "I guess that qualifies you for the STP award money."
Amsoil and most car companies discourage the use of oil additives. Why? There are two big reasons, one, if an additive was necessary Amsoil would have put it in their oil in the first place. Second, all additives on the market have not been tested with all the oil on the market. This means there is no guarantee they will mix properly. Some additives even contain chemicals that will plug up oil filters. That could lead to engine failure. So stay away from the oil additives.