The design of flat-tappet cams makes them especially vulnerable to wear. As the name indicates, the tappet- or lifter - is flat. During operation the surface of the cam lobe slides rapidly over the surface of the tappet, producing high friction and temperatures. The camshaft and lifters are responsible for triggering the precisely tuned movements of the valve train. Here, the use of anti-wear additives becomes crucial. Zinc dialkyldithiophosphate (ZDDP) is a commonly used motor oil additive that provides anti-wear protection and minimizes lubricant breakdown. ZDDP also exhibits mild extreme-pressure protection. As temperatures in an engine rise, ZDDP decomposes, and the resulting chemistry protects critical metal surfaces. Without the protective film barrier provided by ZDDP, the cams and lifters wear from the force of operation, negatively affecting cam and valve operation. Because most V-8 engines of the muscle car era came standard with flat-tappet cams, the problem is prevalent to classic-car and hot rod owners. In these applications, modern oils, such as AMSOIL synthetic motor oils, are capable of providing adequate wear protection after the engine has been broken in. But due to variables like severity of service and level of modification, AMSOIL primarily recommends high-ZDDP oils in these applications, such as Z-ROD® Synthetic Motor Oil. When breaking in a rebuilt or high-performance engine, AMSOIL recommends AMSOIL Break-In Oil, which contains high levels of ZDDP for added protection.