Should I worry about ZDDP?

August 1, 2009 | By Richard Prince


I am currently rebuilding the engine in a 1949 Ford tractor—a fun project! When I picked up the block, etc., from the machine shop, the guy who owns the shop warned me not to use standard, off-the-shelf motor oil for break-in, or I would risk damage to the cam lobes. According to what he said, and what I’ve been able to read on the topic, due to EPA requirements, sometime around 2004, the zinc dialkyl dithio phosphate (ZDDP) content in off-the-shelf motor oil was cut to less than half of what it was in the 1990s.

The “rub” comes in (please excuse the pun) due to the fact that ZDDP is an anti-scuff and anti-wear additive that has been used since the 1930s.

I’ve heard and read from several sources that, if you are running flat tappets and, especially, if you’re building a high-performance engine with higher valve lift and higher valve spring compression force, the chances are pretty good of “cobbing out” your new cam shaft unless you do some research and find an oil with higher ZDDP content for breaking in the engine. What’s your take on it?


Zinc dialkyl dithio phosphate (“ZDDP”) was added to motor oil for many years to improve the oil’s anti-wear protection. But in the early 1980s it was discovered that ZDDP contributed to the accelerated deterioration of catalytic converters and, as a result, oil companies have generally minimized the amount that they add to their products.

Most conventional gasoline engine oils have about .1% ZDDP but some high-performance blends such as Valvoline Race and Pennzoil GT Performance, which are designed primarily for racing and other heavy duty environments, have about twice the concentration of the zinc-based additive.

STP and some other aftermarket additives such as Mechanics Brand Engine Tune Up and K-Mart Super Oil Treatment contain ZDDP.

Of course, the question comes down to whether or not vehicles that are not equipped with a catalytic converter should have an additive containing ZDDP mixed in with the engine oil. And the answer, as usual, is not crystal clear. I have been told that the real role of ZDDP is to add some protection in the unlikely event that there is metal-to-metal contact in the engine and in normal circumstances it is not needed. I have been told that the relatively small amount still present in ordinary oil is sufficient. I have been told that some engines, particularly older ones, will see accelerated wear because of the elimination or reduction of ZDDP.

And I have been told that today’s oils that contain little or no ZDDP still offer better all-around wear protection than older blends that did contain ZDDP.

So whom do you believe? Based on the information I’ve gathered, as well as my instinct, I am comfortable running all of my vintage cars without any oil additives. Where feasible I do use synthetic oil (Mobil 1) but if I believe a switch to synthetic will lead to leaks I stick with conventional oil, without any additives. The fact that the engine oil additive business has a long history of deceptive practices, false claims, and trouble with the law (see for a brief explanation of why the Federal Trade Commission went after the manufacturers of several well-known engine treatment products) does nothing to inspire any confidence in their products in my mind.

In my nearly 30 years of experience in the restoration, OEM, and racing businesses, I have yet to see any engine failure that was actually caused by some shortcoming in a major brand engine oil. If you are concerned about the break-in of your newly rebuilt tractor engine, use one of the above-mentioned Valvoline or Pennzoil motor oils, which contain additional ZDDP.