What Is ZDDP?
To quote one 2005 research study: “The most common antiwear additives used in engines are zinc dialkyldithiophosphates (ZDDPs), which have the chemical formula Zn[S2P(OR)2]2, where R is an alkyl group. ZDDPs have successfully been used for over 60 years, and to date no superior antiwear additive has been developed for use on steel.”
ZDDP has an excellent track record at protecting the sliding metal-to-metal cam lifter interface. Historically, ZDDP has been added to oils in amounts resulting in approximately 0.15% phosphorus and 0.18% zinc. ZDDP protects by creating a film on cams and flat lifter contact points in response to the extreme pressure and heat at the contact point. The film of zinc and phosphorus so formed provides a sacrificial wear surface protecting the base metal of the cam and lifter from wear. In the course of normal service, this conversion of ZDDP to elemental zinc and phosphorus depletes the ZDDP level in the oil.
Studies show that depending on the specific engine and severity of duty, after 2000-4000 miles of operation, the level of ZDDP can drop below that considered adequate to provide wear protection to the cam and lifters. According to the SAE Tech Bulletin # 770087,2 operation of a flat-tappet engine without adequate antiwear additives such as ZDDP quickly leads to lifter foot scuffing and cam lobe wear. Camshafts are typically only surface hardened leaving the core ductile for strength. According to the SAE Bulletin, once cam lobe wear reaches 500 μm (0.020 “), “subsequent wear is usually rapid and catastrophic.”
In order to make engines last in the absence of ZDDP, virtually all IC (internal combustion) engines designed in the last ten years utilize roller lifters. Today, ZDDP has been reduced in practically all automotive engine oils, rendering them less suitable for use with older engines with non-roller lifters. ZDDPlusTM is the ONLY antiwear component which re-establishes the ZDDP levels our classic car’s engines were designed for, while allowing the car owner to use the base oil of their choice.
While some off-the-shelf additives may have some ZDDP, the amount per bottle is small, and when enough is used to get the proper concentration of ZDDP, there is a quart or more of unspecified oil that comes along with it. This dilution of 20% of your oil with unspecified oil also means there is 20% less of the proper additive package. The chart below compares the amount of ZDDP in ZDDPlus™ to GM EOS, a leading additive which claims to provide ZDDP-based wear protection. The ounces of each additive given in the chart is what you would have to add to 5 quarts (160 ounces) to achieve the target phosphorus level listed in red.