Motor oil experts do not recommend adding zinc to engine oil unless the vehicle is a hot rod intended for racing purposes, but also warn of no particular risks associated with adding zinc dithiophosphate to engine oils within strict standards, depending on the age of the engine and its intended use. However, modern motor oil manufacturers produce and sell specialty motor oils with higher levels of zinc already added.
To auto mechanics, the term zinc actually refers to a class of compounds call zinc diakyldithiophosphates that motor oil manufacturers regularly add to their products. There are multiple types of zinc additives, including primary, secondary and ary additives that differ from one another in their activation threshold temperatures. These compounds do not lubricate until heat transforms them into a phosphate glass that protects metal surfaces inside the engine.
Generally, motor oils available on the market after 2010 make use of a zinc additive that is less active than those used in previous years. Certain older engines may perform worse with this particular type of zinc, but this rule typically only applies to highly modified or customized engines. Owners of stock engines who operate vehicles within normal street driving conditions can safely use any oil licensed by the American Petroleum Institute without needing to add zinc.