ZDDP is an anti-wear additive used in motor oil to protect highly loaded sliding surfaces against wear. In most engines, this is the lobes on the camshaft(s). In older engines with flat tappet lifters (both mechanical and hydraulic), ZDDP also protects the bottom of the lifters against wear. In newer engines that have roller cams and lifters, there is much less friction between the cam and lifters, so less ZDDP is required. The same goes for overhead cam engines that use roller cam followers to operate the valves. Other areas inside the engine where ZDDP helps protect parts is at both ends of the pushrods, in the needle bearings inside roller rockers and roller rocker arms, and the piston rings and cylinders at higher engine speeds.
The technical name for ZDDP is Zinc Dialkyl DithioPhosphate. It contains both zinc and phosphorus, though many people simple refer to it as "zinc" when talking about anti-wear additives in motor oil. ZDDP requires both metals working together to prevent wear under high temperatures and load.
The interface between the camshaft and lifters is the highest load point inside a pushrod engine. When the camshaft is opening a valve, the upward movement of the lifter as it rides on the cam lobe has to overcome the pressure exerted by the valve spring. The pressure exerted by the spring may range from 70 to 100 pounds or more when the valve is closed, up to several hundred pounds at maximum valve lift (pressure increases in direct proportion to lift). In performance engines that are built to rev much higher than a stock engine, closed valve spring pressures may be nearly double that of a stock engine, and open pressures can be as high as 600 to 1000 pounds depending on lift! That's a tremendous amount of pressure concentrated across a very narrow strip of the cam lobe and lifter.
If the oil does not contain an adequate amount of ZDDP to protect against wear, bad things can happen to the cam lobe and lifters. Typically, the peaks on the cam lobes that open the valves will wear down and round off, causing a huge drop in power. The bottoms of the lifters may be be scuffed and galled.
All engines require a certain amount of anti-wear additive, be it ZDDP, molybdenum or other ingredients. Back in the 1980s, motor oils usually contained around 1500 parts per million (ppm) of ZDDP.
In the 1990s, the American Petroleum Institute (API) reduced the amount of phosphorus allowed in motor oils to 1200 ppm. This was done to help prolong the life of the catalytic converter and oxygen sensors, both of which can become contaminated over time by exposure to phosphorus in the exhaust. All engines use a small amount of oil between oil changes, so reducing the phosphorus content was seen as a necessary move to prevent emissions failures as vehicles aged.
In 2005, API cut the amount of ZDDP again, this time lowering it to 800 ppm. This includes the current API 'SN' category as well as ILSAC GF-5 motor oils. Their reason for making the change is that modern engines with roller cams or OHC cam followers do not experience the same levels of friction and pressure as flat tapped cams in older pushrod engines. Consequently, today's engines can get by with less anti-wear additive -- which is true.
The trouble is, there are still a lot of older vehicles on the road that have flat tappet cam pushrod engines. What's more, flat tappet cams are still the most economical choice for many performance engines, including classic muscle cars, vintage sports cars, street rods, hot rods, circle track racers, drag racers and even NASCAR. Most of these engines are using high lift performance cams with stiffer than stock valve springs. If conventional motor oils (including synthetic oils and synthetic blends) are used in these engines, cam wear and cam failure can occur.
Most oil companies say that today's motor oils with reduced ZDDP are adequate for older pushrod engines with flat tappet cams -- provided the engines are stock and are driven normally. Others would disagree and recommend adding some extra ZDDP to the oil if you have an older engine or performance engine.
We recommend that you use an aftermarket ZDDP oil additive or a motor oil that is specially formulated with higher levels of ZDDP anti-wear additive for older engines. Many of these oils are marketed as "street performance" oils. Most of these oils are NOT approved for use in newer vehicles with catalytic converters.
Aftermarket ZDDP crankcase additives are available for adding ZDDP to modern motor oils. The dosage is usually designed to increase the level of ZDDP in the crankcase to around 1500 ppm, which most experts say is the optimal level for protecting most flat tappet cams.
Follow the directions on the product and do not exceed the recommended dosage. Overdosing on ZDDP should be avoided because too much ZDDP can interfere with detergents and dispersants, and above a certain level no additional protection is gained up upping the dosage of ZDDP.
Most ZDDP oil additives are formulated for OLDER engines that do not have oxygen sensors or catalytic converters. Most of these products are NOT recommended for late model vehicles with OBD II emission controls.
What happens if you use a ZDDP additive in a newer vehicles that has O2 sensors and a catalytic converter? If you follow the recommended dose on the product and don't overdo it, and if the engine is not using an unusual amount of oil (less than a pint between normal oil changes), it's doubtful the ZDDP will have any negative effects on the oxygen sensors or catalyst.
However, if you continue to use such a product long term (many tens of thousands of miles), it may eventually cause some contamination and shorten the service life of the O2 sensors and catalyst. The O2 sensors would be come sluggish and slow to respond to changes in the air/fuel mixture, causing a drop in fuel economy. The catalyst might lose efficiency at converting exhaust pollutants into CO2 and water vapor, causing an increase in tailpipe emissions. Either of these conditions could be a problem if you live in an area that requires emissions testing and your Check Engine light comes on, indicating an emissions fault. Your vehicle will not pass an emissions test if there are any issues with the O2 sensors or catalyst.
Because diesel engines operate at higher temperatures and loads, diesel oil contains higher levels of ZDDP than motor oils for gasoline engines. Diesel oils and contain more detergents and dispersants to deal with soot and combustion byproducts. Because of this, diesel oils such as Shell Rotella or Mobil Delvac have sometimes been used in performance engines with flat tappet cams. But like gasoline motor oils, the amount of ZDDP allowed in diesel oil has also been reduced to protect exhaust emission controls. Currently, up to 1200 ppm of ZDDP is still allowed in diesel oil.
For serious performance engines that are not driven on the street, specially formulated racing oils that contain higher levels of ZDDP are recommended. Racing oils typically have 1500 ppm or more of ZDDP along with other special friction modifiers and additives to help cut friction and prevent wear in a racing application. The mount of detergent and dispersant in racing oil is usually less than a street oil because the oil is changed much more often.
Using a racing oil on the street is NOT recommended, especially if you think you can get by with normal oil change intervals. Racing oils are designed for short term use only.