Why use high zinc motor oil? ZDDPs Explained

As a classic car enthusiast or high-performance engine owner, you've likely heard about high zinc motor oils. But why are they so important, and why should you consider using them? As a former Ford Motor Company Engineer and current AMSOIL Regency Platinum 12-Star Direct Jobber, I've spent years working with various engines and lubricants, recommending the right oils to hundreds of customers and retailers, as well as deciding on the best oils for my vehicles, equipment, and machinery. Today, I'm going to share my knowledge about high zinc motor oils and why they might be crucial for your engine.

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Understanding ZDDP (Zinc Dialkyldithiophosphate)

At the heart of high zinc motor oils is a compound called ZDDP (Zinc Dialkyldithiophosphate). ZDDP is a powerful anti-wear additive that's been used in motor oils for decades. It works by forming a protective layer on metal surfaces, particularly in areas of high pressure and temperature within your engine. This layer acts as a heat-activated, sacrificial barrier, preventing metal-to-metal contact and reducing wear especially in high-friction environments.

ZDDP additives are heat-activated, meaning they provide wear protection in areas of increased friction. As temperatures rise and surfaces come closer together, ZDDP decomposes and the resulting chemistry protects critical metal surfaces. When parts move during operation, any sliding or rolling motion takes place on top of or within the ZDDP anti-wear film, which reduces metal-to-metal contact.

The Benefits of High Zinc Motor Oil

High zinc motor oils offer several key benefits:

1. Enhanced wear protection: The higher concentration of ZDDP provides superior protection against wear, especially in engines with flat-tappet camshafts.
2. Improved performance in older engines: Classic engines were designed with these higher zinc levels in mind, and using a high zinc oil can help maintain their performance and longevity.
3. Protection for engines with flat-tappet camshafts: These components are particularly vulnerable to wear and benefit significantly from the added protection of high zinc oils.

When to Use High Zinc Motor Oil

High zinc motor oils are particularly beneficial for:

1. Classic cars and vintage engines: These older designs often require higher levels of zinc for proper protection.
2. High-performance and racing applications: The extreme conditions in these engines benefit from the added wear protection.
3. Engines with flat-tappet camshafts: These components are especially reliant on the protective layer formed by ZDDP.

The Evolution of Motor Oil Formulations

In recent years, zinc levels in motor oils have been reduced to protect catalytic converters in modern vehicles. While this change benefits newer cars, it has left owners of classic and high-performance engines searching for alternatives. This is where specialized high zinc oils, like AMSOIL Z-Rod, come into play. High zinc motor oil formulations are particularly beneficial in engines utilizing flat-tappet lifters.

It's important to understand the difference between flat-tappet and roller cams. Flat-tappet cams, common in classic V-8 engines, are more vulnerable to wear due to their design. The cam-lobe slides rapidly over the flat tappet, increasing friction and temperatures. A thin oil film is the only barrier preventing wear, making high-zinc oils crucial for these engines. Roller cams, found in most modern vehicles, have rolling contact rather than sliding contact, reducing the need for high-zinc oils.

FAQs About High Zinc Motor Oils

As an AMSOIL Dealer and former Ford Engineer, I often get questions about high zinc motor oils. Here are some of the most common:

What does high zinc do to an engine?

High zinc content forms a protective layer on metal surfaces, reducing friction and wear. This is particularly important for engines with flat-tappet camshafts and other high-pressure contact points.

Is ZDDP good for your engine?

Yes, especially for older engines or high-performance applications. ZDDP offers superior anti-wear protection and helps extend engine life in vehicles designed for higher zinc content oils. However high ZDDP levels can reduce the performance of catalytic converters, so double check your vehicle's service manual to ensure the OEM doesn't prohibit the use of high zinc engine oils.

How much zinc should be in engine oil?

For older engines or high-performance applications, zinc levels should typically be around - parts per million (ppm). AMSOIL Z-Rod, for example, contains over ppm of zinc. For break-in, AMSOIL Break-In Oil contains even higher levels: 2,200 ppm zinc and 2,000 ppm phosphorus. After break-in, it's recommended to use an oil with at least 1,000 ppm ZDDP in a flat-tappet engine for optimal protection.

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Is zinc good for small engines?

While beneficial for wear protection, most modern small engines don't require high-zinc oils. However, for older or high-performance small engines, a high-zinc oil can be beneficial.

Can you have too much zinc in oil?

While zinc is beneficial, excessive amounts can potentially harm catalytic converters in modern vehicles. That's why it's crucial to use high-zinc oils only in recommended applications, typically older or high-performance engines.

AMSOIL Z-Rod: A Solution for Classic and High-Performance Engines

As an AMSOIL Dealer, I've seen firsthand the benefits of AMSOIL Z-Rod for classic and high-performance engines. Here's why it stands out:

Features of AMSOIL Z-Rod

• Z-Rod is specifically formulated to protect engines in classic and high-performance vehicles. It provides robust anti-wear protection and helps prevent rust during long-term storage.
• AMSOIL Z-Rod contains over ppm of zinc, meeting the needs of engines designed for high-zinc oils.
• Z-Rod not only protects your engine during operation but also during storage periods, making it ideal for seasonal vehicles or show cars.

Best Practices for Using High Zinc Motor Oils

To get the most out of high zinc motor oils:

1. Choose the right oil for your engine: Consult your vehicle's manual or a trusted expert to ensure you're using the appropriate oil for your specific engine.
2. Follow recommended oil change intervals: While high zinc oils offer superior protection, they still need to be changed regularly. Follow the manufacturer's recommendations or consider oil analysis to determine the optimal change interval.
3. Be aware of potential impacts on catalytic converters: If your classic car has a catalytic converter, be mindful that long-term use of high zinc oils could potentially affect its performance.

As we've explored, high zinc motor oils play a crucial role in protecting classic and high-performance engines. The higher concentration of ZDDP provides the necessary wear protection these engines need, particularly for components like flat-tappet camshafts that are prone to wear.

From my years of experience both as a Ford Engineer and an AMSOIL Dealer, I can confidently say that using the right oil makes a significant difference in your engine's performance and longevity. For owners of classic cars, hot rods, or high-performance vehicles, a high zinc oil like AMSOIL Z-Rod can be the key to maintaining your engine's health and performance.

Remember, while high zinc oils are essential for certain applications, they're not necessary or even recommended for all engines. Modern vehicles with catalytic converters are designed to work with lower zinc content oils. Always consult your vehicle's manual or a trusted expert to determine the best oil for your specific engine.

Ultimately, the goal is to match the right oil to the right engine. By understanding the role of zinc in motor oil and the needs of your particular engine, you can make an informed decision that will help keep your cherished vehicle running smoothly for years to come.

Whether you're preserving a piece of automotive history or pushing the limits of performance, choosing the right high zinc motor oil is an investment in your engine's future. And as always, if you have any questions about which oil is right for your vehicle, don't hesitate to reach out to a knowledgeable AMSOIL Dealer like myself. We're here to help you get the most out of your engine, no matter its age or application.

If you want to learn more, please visit our website Benefits Of ZDDP For Older Engines(pt,ja,sr).

I received some really good information on engine oils from a respected professional in the field of engine oil development. He has also written, with another author, a SAE paper titled "How Much ZDP is Enough?". It is SAE document #-01- if you care to locate it and read in greater detail. I have his permission to share a couple of his personal articles on oil myths.


Here is one:

Engine Oil Mythology

Myths are ill-founded beliefs held uncritically by interested groups. Over the years there has been an overabundance of engine oil myths. One was that the only good oils were oils made from “Pure Pennsylvania Crude Oil.” This one got started before the Second World War when engine oil was crude oil with very minimal refining, and crude oil from Pennsylvania made better engine oil than Texas or California crude. With modern refining, almost any crude can be made into good engine oil.

The next myth was that “modern” detergent engine oils were bad for older engines. This one got started after the Second World War, when the government no longer needed all the detergent oil for the war effort, and it hit the market as Heavy-Duty oil. These new detergent oils gave the pre-war cars, which had been driven way past their normal life and were full of sludge and deposits, a massive enema. In some cases bad things happened such as increased oil consumption – the piston rings were completely worn out and the massive piston deposits were the only thing standing between merely high and horrendous oil consumption. If detergent oils had been available to the public during the war, this myth never would have started.

Amazingly, there are still a few people today, 60 years later, who believe that they need to use non-detergent oil in their older cars. Apparently it takes about 75 years for an oil myth to die.

Then there is the myth that new engines will not break-in on synthetic oils. Apparently there was an aircraft engine manufacturer who once put out a bulletin to this effect. Clearly the thousands and thousands of cars filled with Mobil 1 as factory-fill, which have broken-in quite well, should have put this one to rest. However, this myth is only 40 years old, so it has another 35 years to live.

All of these myths have a common theme; newer oils are bad. And this brings us to the latest myth – new “Starburst”/ API SM engine oils are bad for older cars because the amount of anti-wear additive in them has been reduced. This one has gotten big play in the antique and collector car press lately. The anti-wear additive being discussed is zinc dithiophosphate (ZDP).

Before debunking this myth we need to look at the history of ZDP usage in engine oil.

ZDP has been used for over 60 years as an additive in engine oils to provide wear protection and oxidation stability. Unfortunately, ZDP contains phosphorus, and phosphorus is a poison for automotive catalysts. For this reason ZDP levels have been reduced by about 35% over the last 10-15 years down to a maximum of 0.08% for “Starburst”/API SM oils.

Zinc dithiophosphate was first added to engine oil to control copper/lead bearing corrosion. Starting in , a Chevrolet Stovebolt engine with aftermarket copper/lead insert-bearing connecting rods was used for the standard engine oil qualification test. The insert-bearings were weighed before and after test to evaluate weight loss due to corrosion. The phosphorus levels of oils that passed the test were in the 0.03% range.

In the mid-’s, Oldsmobile got into a horsepower war between its Rocket engine and the Chrysler Hemi. Both companies went to high-lift camshafts and both got into camshaft scuffing and wear problems very fast. There were three solutions: 1) better camshaft and lifter metallurgy, 2) phosphating the camshaft, and 3) increasing the phosphorus level from ZDP up to the 0.08% range. In addition, the industry developed a battery of oil tests (“Sequences”), two of which were valve-train scuffing/wear tests.

Knowing that this higher level of ZDP was good for flat-tappet valve-train scuffing and wear, some oil companies, thinking that they were offering the customer additional protection dumped even more into the oil. It was soon learned, however, that, while going above about 0.14% phosphorus might decrease break-in scuffing, longer-term wear increased. Further, at about 0.20% phosphorus, the ZDP started attacking the grain boundaries in the iron, resulting in camshaft spalling.

In the ’s, the ZDP level was pushed up to the 0.10% phosphorus range because it was a cheap and effective antioxidant. The increased antioxidancy was needed to protect the oil in Cadillacs pulling Airstream trailers from thickening to a point where the engine could no longer pump it. Recently, the need for this higher level of ZDP to protect the oil from thickening has been greatly reduced with the introduction of modern ashless antioxidants that contain no phosphorus.

Enough history. Getting back to the myth that “Starburst/API SM oils are no good for older cars, the argument put forth by the myth believers is that while these oils work perfectly well in modern, gasoline engines equipped with roller camshafts, they will cause catastrophic wear in older engines equipped with flat-tappet camshafts.

The “Starburst”/API SM oil standards were developed by a group of OEM, oil additive company, and oil company experts. When developing any new engine oil standard, the issue of “backward compatibility” is always of great importance. Indeed, the group of experts spent a lot of time researching this issue. In addition, multiple oil and additive companies ran “no harm” tests on older cars with the new oils; no problems were uncovered.

Beyond the “no-harm” testing, the new “Starburst”/API SM specification contains two valve-train wear tests. One is the Sequence IVA, which tests for camshaft scuffing and wear using a 2.4L Nissan single overhead camshaft engine with slider finger followers. The wear limits for this test were tightened from those of the previous oil specification, even though the old spec had a higher, 0.10%, phosphorous limit. The second test is the Sequence IIIG, which evaluates cam and lifter wear. For this test, a current-production, GM Powertrain 3.8L engine with the valve train replaced with a flat-tappet system, similar to those used in the ’s, is used. The only reason for using this older valve-train design is to ensure that older engines are protected. All “Starburst”/API SM oil formulations must pass these two tests.

In addition to the protection offered by these two valve-train wear tests and the “no-harm” testing, a review of the knowledge gained over the years in developing previous categories also indicates that no problem should be expected. For example, the new “Starburst”/API SM oils contain about the same percentage of ZDP as the oils that solved the camshaft scuffing and wear issues back in the ’s. They do contain less ZDP than the oils that solved the oil thickening issues in the ’s, but that is because they now contain high levels of ashless antioxidants, which were not commercially available in the ’s.

The oil’s ZDP level is only one factor in determining the life of an older camshaft or a new aftermarket camshaft. Most of the anecdotal reports of camshaft failures attributed to the newer oils appear to be with aftermarket camshafts. Breaking in extremely aggressive aftermarket camshafts has always been a problem. The legendary Smokey Yunick wrote that his solution to the problem was to buy multiple camshafts and simply try them in a slave engine until he found one that survived break-in without scuffing.

Despite the pains taken in developing special flat-tappet camshaft wear tests that these new oils must pass and the fact that the ZDP level of these new oils is comparable to the level found necessary to protect flat-tappet camshafts in the past, there will still be those who want to believe the myth that “new oils will wear out older engines.” Like other myths before it, history teaches us that it will take about 75 years for this one to die also.


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The following is from a presentation he gave to a classic car club.

Are Newer Engine Oils Really Bad for Older Cars?


Miracle Drugs/Additives
* Engine oil additives are like drugs:
- More is not necessarily better
- Take just enough to control the problem
- They have side effects, sometimes fatal
- They often interfere with each other
* Formulating engine oils is a balancing act
- Standardized engine testing is used to get the correct balance

Sequence Tests
* Standardized engine tests for determining oil quality have been used for 50 years
* Currently they are:
- Sequence IIIG; viscosity increase, deposits, and wear
- Sequence IVA; scuffing
- Sequence VG; sludge
- Sequence VIB; fuel economy
- Sequence VII; copper/lead bearing corrosion

Sequence IIIG
* The Sequence IIIG test is based on the current 3.8L GM engine
* The engine is retrofitted with a flat tappet valve-train based on the 3.8L Buick
* This ensures that older engines using wear sensitive flat tappet/push rod valve-trains are protected by ILSAC GF-4/API SM oils

Sequence IVA
* This test is based on an out-of-production 2.4L Nissan engine equipped with a single overhead camshaft and finger followers
* This valve-train is similar to that used on the old Ford 2.3L Pinto engine
* This type of valve-train can be more wear prone than the Sequence IIIG valve-train

ILSAC “Starburst”/API SM
* The current oil standard recommended by US and Japanese auto companies incorporates two camshaft scuff/wear tests that are based on non-roller valve-trains to ensure “backward compatibility” of newer oils
* The ILSAC/OIL Committee, which developed this standard, did not “ignore wear issues in older engines and only worry about catalyst life”

Are newer Engine Oils Really Bad for Older Cars?
There is no reason to believe that newer oils will not protect older cars on the road from premature cam and lifter wear. Lets examine why

Why should newer oils protect older engines?
* The ZDP levels in newer oils are comparable to those needed to solve scuff problems encountered in the ’s with some older engines
* The newer oils must pass two rigorous camshaft scuff/wear tests (IIIG and IVA)
* Somewhat higher ZDP levels were used in oils in the ’s because the ZDP was also being used as an antioxidant
* Ashless antioxidants have reduced the need in today’s oils for ZDP to be used as an antioxidant

Next Question
* Are newer engine oils bad for breaking-in engines rebuilt with flat tappet camshafts?
- The Sequence IIIG and IVA tests start with new cams and lifters with no camshaft pre-lube
* So again the answer is the same: newer oils should protect at least as well as older oils

The Real Question?
* Are newer engine oils bad for breaking-in rebuilt engines equipped with high performance camshafts?

Camshaft Break-in Scuffing
* High performance flat tappet camshaft break-in has always been problematical
* Smokey Yunick said the best way is to order several identical cams, pick the best looking ones, put them in a slave engine, run a break-in, and if one didn’t scuff-use it!

“The devil is in the details”*There are at least 20 different design and production parameters that must be closely controlled to ensure proper camshaft break-in
* Engine oil is just one of them
* Oil tends to get more than its share of attention because the average engine rebuilder can’t do much about things like lifter bore positioning or angularity

“Nothing but the oil changed”
* When something goes wrong, I am always told the oil was the only thing that changed
* Usually many things have changed

You may not like to hear it but…..
There may be no oil in the world that will protect some of these high performance camshafts with excessive lift rates that cause the contact patch to run off the edge of the lifter

What is ZDP?
* Zinc-dialkyl-dithiophosphate
* Abbreviated as ZDP or ZDDP, which ever you prefer
* Forms a polyphosphate tribo film separating the cam lobe from the lifter
* Prevents adhesion of the lobe to the lifter, and break-in scuffing
* Also protects the surfaces from abrasive and corrosive wear

ZDP: the Miracle Drug
* The truly amazing thing is that the ZDP forms this polyphosphate tribo film where it is needed
* A thin film, measured in nanometers, is formed on surfaces as the result of contact pressure and sliding speed
* You only need enough active ZDP to form the film initially and replenish it as it wears away

How Much ZDP is Enough?
* It does not take much ZDP to form these thin films on the wear surfaces
* The ZDP does not “waste itself” forming films on all the surfaces of the crankcase
* ILSAC GF-4/API SM oils contain much more ashless antioxidants, which allow the ZDP to “save itself” for acting as an anti-wear agent instead of an antioxidant

Summary
* The preponderance of dynamometer and vehicle testing indicates that the current level of ZDP in modern passenger car motor oils is more than enough to protect older engines
* If you want to install a high-performance camshaft, invest in a roller-follower cam and lifters