What is zinc oil additive?

According to Tony and Michele Hamer for About.com, zinc, in the form of zinc dialyldithiophosphate, or ZDDP, is added to motor oils to help protect engines. Zinc oil additives are known to help protect against metal-on-metal wear and can also protect against corrosion.

The Hamers note that there has been pressure to eliminate or reduce zinc additives in motor oil because of concerns about human and environmental toxicity. Also, catalytic converters have a shorter lifespan if they are exposed to zinc additives. Modern car engines seem to have a much lower need for zinc additives than classic car engines due to their lower pressure metal-to-metal construction.