Debunking The Urban Legend of ZDDP

The Pros and Cons of ZDDP

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 the classic car hobby is left behind. Now, removing lead from gasoline was not a bad idea. Although it reduced engine knocking and boosted octane ratings, it was poisonous to the atmosphere.

The next blow to the classic car hobby come 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.

Now the oil companies are under pressure to remove zinc dialkyl dithiophosphate, or ZDDP, from engine oils. This article will uncover details about this lubrication additive. See why this change affects classic cars more than new cars.

What Is ZDDP? 

ZDDP, zinc dialkyl dithiophosphate, is a compound developed in the 1940s. Petroleum companies added it to their motor oils to enhance lubrication. It was held in high regard as being the most cost-effective metal on metal anti-wear additive available. The silicate-based fluid got its start in airplane engines. Designers discovered it also worked well in car and truck engines.

Studies found the anti-wear protection with flat tappets and overhead cam lobes impressive. Further investigation uncovered that the benefits applied to any engine part under considerable pressure. In fact, the ZDDP oil additive substantially reduced noticeable wear in any metal to the metal situation.

All tests we’ve reviewed conclude that ZDDP is effective at increasing the longevity of internal engine parts. When properly formulated with base oils, 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.

Cons of ZDDP

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. Unfortunately, it has long-lasting effects when it finds its way into the water table. Proper safety and disposal practices could mitigate this problem.

However, pollution is not the only issue associated with this oil additive. Further, influencing the decision is that catalytic converter lifetimes are decreased by contamination with zinc and phosphates. Hence the drive to decrease the use of the additive by lowering concentration levels. This has now turned into a movement to completely eliminate the chemical compound.

Removing ZDDP from Engine Oils

What happens if you remove ZDDP from engine oil? As petroleum manufacturers decreased the concentration in their products over the last 20 years, concerns began to surface. The main concern is the impact on engine wear in both classic and modern automobiles.

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. Therefore they have lower pressure metal to metal contact and consequently require lower performance additives. Nevertheless, the impact on classic engines was more concerning.

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. Yet others say the replacement lifters did not meet hardness specifications.

This problem is also attributed to lubrication problems 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 we would offer the following observations.

  • Consider using ZDDP as an additive in the motor oil during the break-in period on a rebuilt classic engine. Be careful with the concentration used in your break-in oil. Overdosing can cause increased wear. More is not always better. Always refer to the manufacturer’s specs and measure twice.
  • Consider using classic motor oil, which specifically contains ZDDP already. Valvoline, for example, has a range of products designed for classic car enthusiasts.