In a 2015 article in the Phys.org newsletter, author Evan Lerner reported that the antiwear additive zinc dialkyldithiophosphate, or ZDDP, was essentially discovered by accident in the 1940s. Originally added to prevent rusting, engineers found it increased the antiwear properties of motor oil by some then-unknown mechanism.

So that raises the question: How does phosphorus function as an antiwear agent? Lots of research has been done, and it boils down to this: The element helps form a protective film, which is thicker at higher temperatures and pressures.

There is a hierarchy of anti-friction and antiwear effects related to various chemical agents. The relationship has to do with temperature and pressure in the contact area. At the bottom of the list, when temperatures are not too high, fatty oils, such as methyl oleate, form a weak bond with the metal surface. This is basically how a friction modifier works.

As the temperature gets higher and loads become greater, chlorinated compounds, such as chlorinated paraffins, will interact to form metal chlorides. The only problem with this is that chlorine-containing compounds tend to be very corrosive to engine surfaces, so they arent used in modern engines.

As the heat and load get higher, phosphorus compounds, such as ZDDP, form phosphide protective films.

Last among chemical compounds is sulfur. Sulfur containing compounds react with hot, heavily loaded surfaces to form sulfides. Certainly a combination of sulfur and phosphorus containing chemicals is most efficient at protecting those friction zones.

Antiwear properties of engine oils have traditionally been met through the use of ZDDP. The level of additive ultimately is measured by the level of phosphorus in the oil. Zinc is initially used since it is easier to measure analytically, and the ratio of phosphorus to zinc (0.95 percent phosphorus to 1 percent zinc) determines the amount of phosphorus. That is why you will often hear that high zinc oils are needed for flat tappet engines. In fact, its the phosphorus thats important.

From its accidental beginnings in the 1940s, ZDDP made a name for itself in engine oils for both gasoline- and diesel-fueled engines. In fact, I doubt you could find any engine oil, no matter what fuel is used, that doesnt contain at least some amount of ZDDP.

Beginning in the late 1960s, a new definition began to appear for certain engine oils. They were called universal oils. They were to be used in fleets that had both diesel- and gasoline-fueled engines. Maintenance shops really want to keep things simple: one engine oil for all applications. The same holds true for transmission fluids and gear oils, but thats another story.

At the time, engine design favored lots of antiwear additive with either fuel, so as long as an oil met the engine test requirements for both, you had a universal oil. These oils had ash contents of up to 1.5 percent, zinc (phosphorus) of 0.15 percent and total base number that ran from 8 to 15. There were good reasons for all of these targets, including wear protection, detergency, dispersancy and acid neutralization.

Things rolled along very comfortably, especially for the heavy-duty side of the market, until the introduction of exhaust catalysts, which were initially referred to as catalytic mufflers. That occurred in the mid- to late 1970s and began to put a strain on phosphorus in the oil. However, since the fleets involved had relatively few gasoline-fueled engines where the catalysts were used, the concerns were rather muted.

The subject of limiting phosphorus in engine oils really came up for the first time at an SAE meeting in San Francisco. The late Pete Mysangi of Ford presented data that showed phosphorus and zinc would interact at the temperatures found in the catalyst area to form a glassy material called zinc pyrophosphate. The glass would coat the catalyst surfaces, basically killing the catalyst. This was not a good outcome! At that moment, efforts began in earnest to reduce the phosphorus levels in engine oils that were used in gasoline-fueled engines equipped with exhaust catalysts.

Id like to digress at this point to share some insights from a field test I was involved with in the late 1970s. The new API Group II base stocks from Sun Oil Co.s Puerto Rico refinery (since closed) presented an opportunity to step out into new additive technology. Edwin Cooper (now Afton Chemical) went to work on a modern engine oil additive package, which was formulated to provide a low sulfated ash (0.5 percent) and low phosphorus (0.05 percent) finished oil for the Puerto Rican base stock slate. The formulation we tested was an SAE 10W-30.

At any rate, we set up a field test in some Ford delivery vans with 350 CID V-8 engines. These vans were in mixed service, and oil change intervals of 7,500 and 15,000 miles were selected. After 60,000 miles of operation, the engines were evaluated and looked great. The fly in the ointment, however, was that we could never pass the Sequence IIIC engine test for oxidation. Turns out that the IIIC had an appetite for sulfur in the formulation, either in the base oil or in the additive package. With the improved additive technologies available today, it might have been successful.

Meanwhile, API heavy-duty engine oils were adjusting to changes in diesel fuel sulfur content. As sulfur went down, resulting in less acid formation, the need for a higher TBN was relaxed. Phosphorus continued to be important for wear protection. At that point, we had engine oil categories moving along on a slowly diverging pair of tracks.

Phosphorus limits were first introduced with the API SH passenger car engine oil service category. API SH and ILSAC GF-1 both set a maximum limit of 0.12 percent. This track continued with API SJ and GF-2 at 0.10 percent maximum, followed by API SL and GF-3 also at 0.10 percent and API SM and GF-4 at 0.08 percent. These limits were only set on the SAE viscosity grades that were found in fuel economy-sensitive applications. The higher viscosity grades had no limit on phosphorus.

Meanwhile, the heavy-duty categories were advancing on diesel-fueled engines with emissions controls not based on catalytic converters but with operational adjustments to lower particulate emissions, and later with special traps for particulates in the exhaust. However, the universal oils were still an important part of the oil marketers bag of tricks. In fact, most oil marketers touted their universal oils as top of the line products.

When API SJ was introduced, the first real issues began to surface. As a way to work with heavy-duty engine oils, the following footnote from API 1509 (Engine Oil Licensing and Certification System), Annex G, appeared in the S service category description:

For all viscosity grades: If CH-4, CI-4 and/or CJ-4 categories precede the S category and there is no API Certification Mark, the S category limits for phosphorus, sulfur and the TEOST MHT do not apply. However, the CJ-4 limits for phosphorus and sulfur do apply for CJ-4 oils.

However, when API CK-4 and FA-4 were introduced in 2017, the footnote added the following text:

This footnote cannot be applied if CK-4 or FA-4 is also claimed.

In other words, if you want to claim API CK-4 or FA-4, you have to meet the phosphorus limits for the relevant S category. There are no more waivers allowed. In practical terms, this means that to make a true universal oil, you must meet all parameters of both categories, which has never been done. Bottom line: Phosphorus levels are now set by API S category limits for the most recent, and future, C categories if universal oil claims are made. High viscosity grades (e.g. SAE 15W-40) dont have phosphorus limits.

This is the phosphorus exemption that is making headlines. Will phosphorus be eliminated? The answer to that question is obviously no. Will phosphorus be limited? Here, the answer is yes.

Initially, the impact will be on viscosities that are covered for passenger cars, which include such grades as SAE 0W-16, SAE 5W-20 and SAE 10W-30. There are already SAE 5W-20 heavy-duty engine oils in use in Europe. API FA-4 engine oils are definitely affected. In addition, API CK-4 SAE 10W-30 will be included, and with PC-12 on the horizon, the need for catalytic converters on diesel-fueled engines will likely emerge and create more restrictions on phosphorus use.

Looks like fun times ahead for formulators!

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society, ASTM International and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.