Motor Oils for Classic and Vintage Cars

By Thomas F. Glenn, President PQIA

The Petroleum Quality Institute of America has from time-to-time encountered motor oils containing high levels of a multifunctional oil additive called zinc dialkyldithiophosphate (ZDDP). This additive contributes anti-wear, anti-oxidation, and corrosion inhibition properties to the oil. ZDDP has been used in motor oil formulations almost universally for decades, and oils with high levels of ZDDP are often promoted as being better suited for engines in older cars with “flat-tappet” lifters. Such high levels of ZDDP, however, typically results in these specialty oils not conforming to current API specifications.  Current API specifications, designed for modern vehicles, limit the level of phosphorus in a motor oil in order to protect catalytic converters.

So, what’s the story with motor oils currently in the market that are purposefully formulated with high concentrations of ZDDP, and where are they used?  

To answer these questions, it helps to have a basic understanding of additive technology and how it has changed over the years.

The wide spread use of motor oil additives goes back to the 1940s with the introduction of antioxidants, detergents and dispersants to combat sludge and deposit formation in early engines. Over time, other functional additives were introduced to further improve performance, including additives to inhibit wear, rust and corrosion, oxidation and foaming, as well as to modify viscosity, depress pour point, and reduce friction. Since ZDDP is multi-functional and cost effective, it grew to become a workhorse additive in motor oils worldwide, and the percentage of ZDDP in motor oils increased for several decades since its first use.

The levels of ZDDP, however, started to decline with the introduction of the API SH Service Category. This change in direction was primarily due to concerns that the phosphorus in the ZDDP additive was responsible for reducing the usable life of a vehicle’s catalytic converter. With that, specifications were put in place to limited the amount of phosphorus (effectively the ZDDP) in passenger car motor oils. The first change was seen in 1993 with API SH/ILSAC GF-1 when a phosphorus limit was set at 0.12% maximum. Although this was not a significant departure from levels formerly in use, the maximum was lowered to 0.10% for API SJ/ILSAC GF-2 in 1996, and reduced again in 2004 when the maximum was set at 0.08% for the most commonly used viscosity grades of API SM/GF-4 oils. In addition, a minimum phosphorus level of 0.06% was set to assure an adequate level of anti-wear properties for both modern and older engines.  

Due to wear related concerns, some owners of vintage and classic cars, muscle cars, and other vehicles with flat-tappet lifters prefer engine oils with higher levels of ZDDP. Although current API/ILSAC oils are backward compatible (meaning they are suitable for use in all previous API Service Categories), a notable number of these car owners believe that higher levels of ZDDP provide additional wear protection for their engines where catalytic converter performance is not a concern. To satisfy the motor oil preferences of these vehicle owners, some lubricant marketers offer motor oils formulated for use in older and high-performance engines with flat-tappet lifters. Typically, these oils are labeled with such words as “vintage cars” or “hot rod & classic,” and often state that they contain high levels of zinc and phosphorus.

Although there is a market for these types of specialty motor oils, they present a challenge to the oil industry and consumers. The challenge resides in the fact that some make no claims as to the specifications they meet. As such government agencies and industry bodies like PQIA cannot readily assess the suitability and quality of these motor oils.

This is not to say specialty motor oils like those formulated for use in classic and muscle cars pose a performance concern simply because they do not display a performance specification.  Instead, it means that a consumer has little to rely on other than their trust in the manufacturer and/or retailer when buying motor oils that do not claim to meet any performance specifications (i.e., API, ACEA, OEMs).  Furthermore, although these motor oils may be quality products, there are labeling regulations in place to help protect consumers when choosing a motor oil. These regulations require that motor oils display at least one recognized industry specification. For more on labeling requirements for motor oils see NIST HB-130.

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Please tell me How does ZDDP get to the catalytic converter ? Unless the engine in question burns a lot of oil.


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