A zddp additive to synthetic oil gives your engine the extra lubrication it needs during the critical start-up period. This is a short window of time when an older car’s engine requires more lubrication than modern-day non-zinc motor oils can provide. Zinc adds a protective layer over critical metal surfaces to keep them from micro-welding together as the engine warms up. It does essentially the same job as tetraethyl lead did for exhaust valves and seats before it was removed from gasoline to reduce air pollution.

The zinc in zddp is actually an anti-wear additive called Zinc Dialkyldithiophosphate (ZDDP). It works as a temporary lubricant when mating metals make contact. The temperature of the metallic surface rises during operation, causing a chemical reaction with the ZDDP additive. This chemistry creates a layer of lubrication between the surfaces, reinforcing the film produced by the existing motor oil.

Synthetic motor oils have lower friction due to their chemical structure and can handle higher operating temperatures than mineral oils. But they don’t contain enough ZDDP to give an older car the protection it needs in those crucial first moments of start-up when a new engine’s pistons, camshafts and bearings are moving with extreme force and speed.

Some companies claim you can solve the flat-tappet camshaft wear problem by pouring in a can of ZDDP additive to your oil. However, a tribologist at Driven Racing Oil warns that mixing an acidic additive like ZDDP into high-detergent oil can cause a chemical reaction and clog the lubrication-carrying crevices in cylinder walls.