No, AMSOIL motor oils are motor oils. It is not an additive that you add to your current oil, but is a replacement for your current motor oil. Because it is created in the lab, it does not contain the impurities that petroleum motor oils do. This allows it to do a better job of protecting your engine, and does so for a much longer time than petroleum oils do.
Because AMSOIL does not contain the impurities of petroleum oils, it can last up to 25,000 miles with the Signature Series oil. AMSOIL may be good for even longer intervals if monitored with oil analysis.
In regards to aftermarket oil additives, please do not fall for them. Even if you do not decide to use our oils, don't waste your money on aftermarket additives. Have a look at this article for more information.