Debunking the Urban Legend of ZDDP

The Pros and Cons of This Motor Oil Additive

Side view of female mechanic checking oil with dipstick outside auto repair shop.
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As time marches on in the automotive world, it often feels like classic car owners are being left behind as more and more emissions regulations come into play. Take, for instance, the ongoing debate over ZDDP, a popular anti-wear motor oil additive, whose use has been severely curtailed for decades. But is it really a danger?

No Co2 for You

Certainly, removing lead from gasoline was a good idea. Although it reduced engine knocking and boosted octane ratings, classic car enthusiasts sucked it up because it was poisonous to the air we breathe. The next blow to hobbyists came in the form of ethanol fuels. This type of gasoline works great for a daily driver. However, you might want to seek out ethanol-free fuel if you store an automobile for more than a couple of months.

When the EPA turned its attention to a popular lubricant, zinc dialkyl dithiophosphate, or ZDDP, that was a bridge too far. For decades ZDDP was the gold standard for engine protection, and many classic car owners are afraid that it might eventually be banned.

How ZDDP Works 

Zinc dialkyl dithiophosphate is a silicate-based lubricating compound developed in the 1940s that was first used in airplane engines. It wasn't long before petroleum companies added it to their motor oils to inhibit wear in automotive engines. It was considered the most effective metal-on-metal anti-wear additive available.

As a car's engine heats up and engine parts begin to make contact, so too does ZDDP heat up. As it becomes volatile, it breaks down into a wear-protecting phosphate compound that coats and protects the engine's metal surfaces, thus reducing the kind of metal-to-metal contact that can make an engine seize up. This is especially important in high horsepower engines, like those found in classic and racing cars. As an additive, ZDDP is also known to have antioxidant and corrosion resistant properties. These are useful qualities for classic car enthusiasts since their engines can sit for extended periods.

Toxicity Concerns

Over the last 40 years, there has been considerable pressure to reduce the use of ZDDP in motor oil applications because of long-term toxicity concerns. The chemical compound is especially toxic to aquatic wildlife and has long-lasting effects when it finds its way into the water table. While proper safety and disposal practices could mitigate this problem, pollution is not the only issue associated with this oil additive.

Bad for Catalytic Converters

The same phosphate compound that protects a car's engine is on the other hand detrimental to its catalytic converter, which in modern cars is the part responsible for reducing carbon emissions. Over the past several decades, the EPA has reduced acceptable levels of carbon emissions while increasing the amount of miles guaranteed by a catalytic converter from 50,000 to 120,000 or ten years. As a result, the automotive industry has developed motor oils with ever-decreasing concentrations of ZDDP. This has now turned into a movement to completely eliminate the chemical compound.

Removing ZDDP from Engine Oils

It is now clear that modern passenger car engines are quite different in their need for ZDDP. Many are multi-valve overhead cam engines with lower spring pressures. Those modern engines that still use an overhead valve arrangement use roller lifters instead of flat tappets. They therefore experience reduced metal-to-metal contact and consequently require lower performance additives.

The impact on classic car engines is a different story. There have been reports about problems with rapid engine wear. These included the total destruction of the camshaft and lifters in freshly overhauled engines. Some have blamed this problem on poor quality rebuilds, while others say the replacement lifters did not meet hardness specifications.

This problem is also attributed to reduced lubrication during the engine break-in period. The benefits of ZDDP are especially important for camshafts and lifters during the first few hours of operation. Therefore, it makes sense that the excessive wear and destruction of parts will show up in recently overhauled engines well before we see it in higher-mileage motors.

Solution to the Oil Additive Problem

Whether you own a 1970 Chevrolet Chevelle Super Sport with a 454 cubic inch muscle car engine or a 1948 MG TC series roadster, you want your engine to last as long as possible. So here are some of our conclusions. These types of problems are never simple, but while you still can, consider adding ZDDP to your motor oil during the break-in period on a rebuilt classic engine. But be careful with the concentration. Overdosing can cause increased wear. More is not always better. Always refer to the manufacturer’s specs and measure twice. You can also purchase classic motor oil, which specifically contains ZDDP already. Valvoline, for example, has a range of products designed for classic car enthusiasts.