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ZDDP - Debunking The Motor Oil Additive Urban Legend


ZDDP - Debunking The Motor Oil Additive Urban Legend



ZDDP, Zinc dialyldithiophosphate, is a compound that was introduced over 70 years ago and regularly added to motor oils. Its claim to fame is that it was the most cost effective metal on metal anti-wear additive…….and that is no legend. The compound was originally developed for use in airplane engines, but very quickly was found to be effective in car and truck engines for the anti-wear protection, in particular associated with flat tappets, overhead cam lobes, buckets and followers and the associated lifters where there is considerable pressure generated at the metal to metal interface and wear of the surface is known to be prevalent.

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All tests that we’ve reviewed conclude that ZDDP is effective in moderating the wear, when formulated properly with base stock oils. ZDDP is also known to have anti-oxidant and corrosion resistant properties which are very useful in preventing aging of the internal combustion engine.

Over the last 40 years there has been considerable pressure to reduce the use of ZDDP in motor oil applications because of long term human toxicity concerns, and the fact that they are considered to be toxic to aquatic wildlife with long lasting effects. This can be alleviated with proper safety and disposal practices.

Further, influencing the decision is that catalytic converter life times are decreased by contamination with Zinc and Phosphates, and hence a drive to decrease the use of the additive to lower concentrations and in some cases its elimination.

As engine oil manufacturers decreased the concentration of ZDDP in motor oils over the last 20 years concerns became apparent about the impact on wear in engines, both classic and modern. It is now clear that modern passenger car engines are quite different in their need for ZDDP. Many are multivalve overhead cam engines with lower spring pressures. Those modern engines that still use an overhead valve arrangement use roller lifters instead of flat tappets and hence have lower pressure metal to metal contact and consequently require lower performance additives.

However the impact on classic engines was more concerning. There are reports from several years ago that problems were manifested in the rapid wear and almost total destruction of the camshaft and lifters in freshly overhauled engines. Some have blamed this problem on poor quality rebuilds, and also that the replacement lifters which were not meeting hardness specifications. But this problem is also attributed to the appearance of lubrication problems during the “run in” or “break in” period. The benefits of ZDDP are, after all, especially important during the break-in period for camshafts and lifters, and 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.

So these are some of our conclusions. These types of problems are never simple, but as a result of our reading we would offer the following observations:

  1. Consider using ZDDP as an additive in the motor oil during the run in period on a rebuilt classic engine. We are presently doing this on our rebuilt Jaguar 3.4Litre engine for the Mark 2.
  2. Be careful with the concentration of ZDDP used in your break in oil, as over dosing can cause increased wear – more is not always better! Always refer to the manufacturers specs and measure well.
  3. Consider using in your classic, motor oil which specifically contains ZDDP. Valvoline, for example, has a range of products as do. If you prefer, add ZDDP to your normal engine oil, making sure that you measure properly and achieve the manufacturers recommended concentration levels.

And remember, the ZDDP additive can help ensure a happy and long lived motoring.

Suggested Reading: Marvel Mystery Oil for Your Classic Car

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