ZDDP
Zinc dialkyl dithiophosphate, a critical anti-wear engine oil additive that protects flat-tappet camshaft lobes.
ZDDP, which stands for Zinc Dialkyl Dithiophosphate, is a chemical compound containing zinc and phosphorus widely used as an anti-wear additive in motor oils. It works by creating a microscopic, sacrificial chemical layer on metal surfaces that prevents metal-on-metal contact under extreme loads and boundary lubrication conditions.
ZDDP is particularly critical for protecting flat-tappet camshafts and lifters. Because flat-tappet cams experience high sliding friction between the lobe and lifter face, they require high levels of ZDDP to prevent the cam lobes from wearing down (wiping a lobe). Modern passenger car motor oils have reduced ZDDP levels to protect catalytic converters from phosphorus poisoning, which can degrade emissions systems over 100,000 miles.
Using modern low-ZDDP oil in a classic engine with a flat-tappet camshaft can result in rapid, catastrophic camshaft wear within a few hundred miles of operation. To prevent this, classic car owners and engine builders add a separate ZDDP supplement during oil changes or use specialized high-zinc motor oils. Modern overhead-cam engines with roller rocker arms do not require high ZDDP levels because they use rolling rather than sliding contact.