Proper Use of ZDP in Engine Oils

By John Martin, retired Shell engineer

Zinc dithiophosphate or Zinc dialkyl dithiophosphate (ZDP) is to engine oils what Tetraethyl Lead is to gasoline. Both are highly specialized chemistries which provide significant performance boosts to the fluids they are placed in. ZDP is the most effective extreme pressure (EP) agent known to oil formulators.

But you can’t just dump these chemicals into any oil and expect maximum performance benefits.

Oil Being Poured

ZDP (often called ZDDP or Zinc) is a highly developed family of chemicals used to give engine oils extremely good protection against metal-to-metal contact, which maximizes valve train longevity. One can’t expect to gain significant increases in performance by just purchasing a ZDP additive and dumping it into a motor oil.

When I was racing Funny Cars years ago, I religiously dumped a can of General Motors (GM) Engine Oil Supplement (EOS) into every oil change in our Funny Car, because I knew it contained ZDP. I was certain the ZDP in EOS would result in much-improved camshaft and lifter durability.

One weekend we ran out of EOS while we were at the track, so I was forced to soldier on without it. Upon subsequent teardown and inspection of that engine the following week, no differences in cam and lifter durability were observed.

Had I been the victim of a marketing ploy? At that time I learned that just because automobile and component manufacturers build engines, it doesn’t mean they know everything about engine oil chemistry and additives. Only engine oil formulators do, and there aren’t that many of them out there.

Why can’t the average racer simply purchase a ZDP supplement and significantly improve the cam and lifter durability of his oil? The primary reason is that the ZDP molecule is a complex chemical in which a metallic substance has been combined with a hydrocarbon to make the metallic material soluble in oil, and dissolving the ZDP into the oil is the key to maximizing ZDP performance.

ZDPs were discovered in the 1940s. They were responsible for allowing increased camshaft lifts that high-powered piston aircraft engines needed to produce sufficient horsepower to outrun their enemies. At the time ZDP was discovered, I doubt that researchers fully understood how it functioned. ZDP forms a sacrificial film on the surfaces of the components it is intending to protect. This film slowly wears away, sacrificing itself instead of allowing metal-to-metal contact.

This functionality allowed engine designers to increase valve lifts significantly to improve engine breathing and valve spring pressures accordingly to prevent valve float on more aggressive camshaft profiles. Over the years many different types of ZDPs have been developed as researchers better understand their properties. Today, ZDP compounds are the result of years of research, performance testing and solubility studies by additive manufacturers.RACE GROUP

At first all ZDPs were very chemically active. They solubilized into the oil quickly and immediately began protecting vital surfaces. However, researchers discovered that very active ZDPs became rapidly depleted and were only able to protect engine components for a short period of time.

The Seq. IV dynamometer engine test was developed to make certain that future, less active, ZDPs were developed to offer engine protection which lasted longer than just a few moments. Today additive manufacturers have even developed ZDPs which protect exhaust emissions equipment (catalytic converters and particulate traps) better than previous ZDPs, and these new ZDPs are found in the current API SN specification motor oils.

Over the years researchers and lube oil blenders discovered that a good ZDP could be rendered practically useless by improper blending of the lubricant.  Most ZDPs activate in a distinct temperature range. Different ZDPs activate at different temperatures. Temperature activation is designed to occur at those temperatures experienced by the camshaft/lifter interface. If ZDP is added to cold oils, it may not go into solution at all and precipitate out into the crankcase. If ZDP is added to overly hot oil, the ZDP may activate in the crankcase (or sump) before getting to the valve train components it is supposed to protect.

Good lubricants are mixtures of 10-12 separate chemicals which are blended together at specific temperatures in a defined order.

ZDP is added only at that point where blenders can be certain that the oil will no longer exceed the ZDP activation temperature. I don’t know what those temperatures are, and I’ll bet you don’t know either.


Another blending concern with ZDPs is the oil you are adding it to. As I said previously, ZDP forms a sacrificial film on the surfaces of the components it is intended to protect. Some highly compounded (high levels of additives) oils such as diesel engine oils contain considerable quantities of detergents to protect the engines from the harmful effects of soot (unburned Carbon). Detergents act in much the same way as ZDP. They must be absorbed on the surface to do their job.

The detergents in high-detergent oils often compete with the ZDP for the cam and lifter surfaces, resulting in those components having insufficient metal surface for the ZDP film to be effective. ZDP must be kept separate from detergents during the blending process. That’s why most racing oils contain low levels of detergents – the ZDP is more effective for better valve train protection.

The safest way to ensure maximum camshaft and lifter protection is to purchase engine oils specifically compounded for your application. It’s always best to let an experienced oil formulator and blender put your oil together for you. It may cost a bit more, but it will be well worth it in the long run. A properly formulated racing oil will not only protect cams and lifters, but it can also extend the life of valve springs. Saving a racer a set of valve springs can more than offset the extra cost of a properly formulated oil.