Does Castrol GTX Have Zinc [Explained A-Z]

Using an oil that is high in Zinc is essential for hot rods and classic cars. As Castrol GTX is one of the most popular choices of oil, you’re likely interested in giving it a go. But it needs to have sufficient Zinc to be used in older engines. So, does Castrol GTX have zinc?

does castrol gxt have zinc

The answer is yes, Castrol GTX oils have zinc in them. However, it’s only the Classic Castrol GTX 20W50 oil that has enough zinc for older engines. The modern Castrol GTX oils have Zinc but it’s not enough for classic cars.

Keep on reading to find out all about it in detail. I’ve also talked about the Castrol Edge oils in case you’re thinking of them as an option as well. So, let’s dive right in!

Does Castrol GTX Have Zinc [Answered A-Z]

The Castrol GTX oil has been around for a long time. It’s still a popular oil to this day. But if you want to get this oil, there are two options – the regular GTX and the classic GTX oil.

castrol gtx

First, you’d want to know that both these oils have zinc in them. However, the amount of ZDDP in both oils is significantly different from one another. So, let’s find out the exact ZDDP content in them.

Let’s start with the classic Castrol GTX first as it’s more in line with other high-performance and racing engine oils. Castrol claims that this oil has 1300 PPM Zinc and 1200 PPM Phosphorus in the Classic GTX 20W50 oil.

castrol gtx classic oil

However, I’ve done independent testing on this oil. According to my results, I’d say that the oil has around 1500 PPM Zinc and 1300 PPM Phosphorous. So, take the value that Castrol claims with a grain of salt.

Now, let’s look at the regular Castrol GTX oil. I haven’t found a data sheet that tells the exact amount of ZDDP in it. But I’ve also done testing on it. The amount of Zinc is nearly 900 PPM and there’s around 700 PPM Phosphorus in the Castrol GTX oil.

So, you can clearly see that the classic GTX oil is much higher in Zinc. In fact, the classic oil has almost double the Zinc compared to the modern oil. But there are many other components that make these two oils different from one another. Keep on reading to know more about them.

What Is The Difference Between Castrol GTX and Castrol GTX Classic?

I’ll talk about the main differences between them. I am going to compare the 20W50 viscosity variants of both these oils now.

1. Regular Castrol GTX Oil

Regular Castrol GTX oil is an API SP-rated oil. If you don’t want to take my word for it, here’s a picture of the label on the back of the oil bottle:

castrol gtx oil api rating

You already know about the Zinc content in the regular Castrol GTX oil. However, learning about the composition of other additives will help you better determine if this is a good oil.

There is about 57 PPM of Molybdenum and 134 PPM of Boron in this oil. Besides, there are also detergents like 1000 PPM Calcium and nearly 435 PPM Magnesium in the Castrol GTX oil.

The oxidation value of this oil is around 4.4. Considering all these things, it’s safe to assume that it’s a Group 2 base stock oil. The additives are right in line with what’s commonly seen in the other API SP-rated oils.

When it comes to oils that are of viscosities of 10W30 or lower than that, the highest Zinc allowed in them is about 800 PPM. If you go over that, the oil won’t be API-approved.

But you should know that I have been talking about these specifications for the 20W50 variant Castrol GTX oil so far. In these oils, you’re allowed to put up to 1200 PPM of Zinc to comply with the API SP rating.

Despite the ability to put more Zinc, Castrol didn’t do it. When you break down the additives in this oil, there’s nothing in them to suggest that it’s a good racing oil. So, it’s not a high zinc motor oil at all. To be honest, it’s a pretty basic passenger car oil. It just happens to be in a 20W50 package. 

2. Classic Castrol GTX Oil

If you look at the label of the Classic Castrol GTX oil, you’ll notice that it’s marketed as a high-zinc oil. And you already know it’s high in Zinc and the label does reflect the truth about the oil. It’s also an API SJ-rated oil. So, the formulation of additive packages in this oil is much different.

It has way more detergents compared to the regular oil. There’s about 3700 PPM Calcium and 11 PPM Magnesium in it. The oxidation value in this oil is 6. It’s possible that Group 1 base stock is used in this oil for which the oxidation is a little higher.

There’s also more Molybdenum in Classic Castrol GTX oil. All these things together make it the better option if you’re looking for a robust oil.

I understand this information can be a little confusing for you and it may be hard to keep track of the additives. So, here’s a simple chart I’ve made to help you figure out the amount of additives in both these oils:

Name Of AdditiveAmount In Classic Castrol GTX OilAmount In Regular Castrol GTX Oil
Zinc1500 PPM900 PPM
Phosphorus1300 PPM700 PPM
Molybdenum123 PPM57 PPM
Boron00 PPM134 PPM
Calcium3700 PPM980 PPM
Magnesium123 PPM434 PPM
Sodium00 PPM00 PPM

Why Is High ZDDP Important For Older Engines?

The design of the older engines from the modern ones is different. Most of the older engines of your classic cars have a flat tappet camshaft. Those engines are much more likely to undergo severe engine wear.

Modern engines incorporate a roller camshaft design. So, these engines are not at a high risk of wearing out soon. So, they don’t need the same level of protection that the older engines do.

ZDDP plays a critical role when it comes to protecting older engines from engine wear. Another common trend in such engines is that they create a lot of friction. So, generally, the oxidation is also higher in them.

An engine oil with a high quantity of ZDDP also acts as an oxidation inhibitor. So, a high Zinc engine oil is essential for push rods, flat tappet engines, high performance, and racing vehicles.

The ZDDP creates an additional layer of protection to keep the metal parts from rubbing on each other. That’s how it reduces engine wear. A high Zinc engine oil should also have less foaming and good resistance to thermal breakdown.

Now, you should have a clear idea why high Zinc oil is needed in older engines. It’s a must for engines that were made in the 1980s were before. Even for rebuilt engines, you’d need a high Zinc engine oil. And the classic Castrol GTX 20W50 oil is a good choice for this job.

How Many Types Of Castrol GTX Oils Are There?

I’ve talked about the two most popular types of Castrol GTX oil. And even in that, I focused on the 20W50 variant. But these types of oils are available in more than one variant.

You can find the regular Castrol GTX oil in viscosities 10W30 and 10W40 along with the 20W50 variant. However, the classic GTX oil can only be found in the 20W50 viscosity that I’ve talked about so far.

On top of that, there are many other types of Castrol GTX oils. Let’s look at the other types of Castrol GTX oils.

1. Castrol GTX Fully Synthetic Oil

There is a Castrol GTX fully synthetic oil. Castrol claims that it gives 1.3X more protection against sludge build-up and 6X more protection against engine wear. This line was introduced by Castrol after discontinuing the GTX MAGNATEC motor oil line.

castrol gtx fully synthetic oil

This oil has better oxidation and thermal stability as it’s a fully synthetic oil. It also prevents viscosity breakdown. This fully synthetic oil is available in 0W20, 5W20, and 5W30 variants.

I looked at the data sheets to see if I could find the amount of Zinc for them. But that wasn’t revealed in them. But as the oils are rated between SP and SN Plus, it’s safe to say that none of them are high Zinc motor oils.

2. Castrol GTX High Mileage Oil

There is also a Castrol GTX high mileage oil. It’s a synthetic blend of engine oil. I haven’t found a data sheet that reveals the ZDDP content of the oil. But I have to assume that there won’t be much Zinc or Phosphorus in it.

castrol gtx high mileage oil

That’s because the oil is marketed as having been engineered to protect the emission system. The use of Phosphorus can damage the catalytic converters. So, Phosphorus replacement technology has been used in this oil to eliminate the need to use Phosphorus.

There are also additives to prevent engine wear and sealants to prevent leaks. You can find this high-mileage oil in viscosities 5W20, 5W30, 10W30, 10W40, and 20W50.

3. Castrol GTX Ultraclean Oil

Next up is the Castrol GTX Ultraclean oil. As the name suggests, the big claim to fame of this oil is that it keeps the engine up to 1.4x times cleaner than the industry standard.

castrol gtx ultra clean oil

This oil works with a double-action formula. The first part is to clean the sludge that blocks the engine components. The second part is to prevent the formation of new sludge.

The Zinc content in this oil is also not mentioned in the data sheets. But from the API ratings of this oil which are SP, SN, and SN+, you already know that it’s not a high Zinc oil. You can find this oil in viscosities 0W20, 5W20, and 5W30.

4. Castrol GTX Diesel Engine Oil

The final oil in this Castrol GTX lineup is the diesel engine oil. This oil is available in 15W40 viscosity only.  It can be used in diesel engines that have to meet standards like API CK-4, CJ-4, or the ones below them.

It’s a versatile oil that can be used in heavy-duty engines and pickup trucks. The date sheet didn’t reveal the amount of ZDDP. But through some testing, it can be estimated that there’s nearly 1000 PPM Zinc and 900 PPM Phosphorus in this oil.

Are Castrol Edge Oils Different From Castrol GTX Oils?

Apart from the Castrol GTX motor oils, Castrol has a different lineup of oils called Castrol Edge oils. The main difference between these two types of oils is that the Castrol Edge oils are all fully synthetic oils. But the GTX oils are a mix of conventional, synthetic blends, and synthetic oils.

Castrol has used what they call Fluid Titanium Technology to enhance the Castrol Edge oils. It helps make the oil stronger and lowers the friction between the different engine parts. Let’s take a quick look at all the different types of Castrol Edge oils.

1. Castrol Edge Unlock Maximum Engine Performance Oil

According to Castrol, this is their strongest engine oil. They also say, “Castrol® EDGE® with Fluid TITANIUM Technology, is Castrol’s strongest oil and the natural choice for drivers who want the best combination of sludge protection, wear protection, and maximum engine performance. Unlock your engine’s maximum performance with every drive”.

This oil is also known to give 6X better wear protection and 10X better performance at high temperatures. You can also drive your car for 10,000 miles before needing another oil change.

castrol edge max performance oil

The Castrol Edge oil meets or surpasses the API SP, SN, SN Plus, and ILSAC GF-6 requirements. You can find this oil in viscosities 0W16, 0W20, 5W20, 5W30, 5W50, 10W30, and 10W40.

2. Castrol Edge High Mileage Oil

There is a Castrol Edge High High Mileage oil just like there was a Castrol GTX high mileage oil. This oil is ideal for vehicles that have already been driven for more than 75,000 miles.

It’s a carbon-neutral oil where Phosphorus Replacement Technology has been used. So, it won’t take a toll on the emission system and also offers great protection. This oil is available in the viscosities 0W20, 0W30, 5W30, 10W30, and 10W40.

3. Castrol Edge Extended Performance Oil

The main benefit of using this oil is that you can drive it for 20,000 miles or a year without changing it. This Castrol oil performs just as well as the other Edge oils by offering 6X wear protection and 10X better performance at higher temperatures. It also improves oil film strength by 30%.

castrol edge extended performance oil

The 0W variant of this oil also helps improve the fuel economy. You can find it in viscosities 0W20, 5W20, and 5W30. It meets the ILSAC GF6, and API SN, SN Plus, and SP requirements.

The data sheet provided by Castrol doesn’t reveal the amount of ZDDP in this oil. However according to Bob Is The Oil Guy, the 5W30 variant of this oil has nearly 730 PPM Zinc and 600 PPM Phosphorus.

How To Determine The Right High Zinc Engine Oil For You?

There are many benchmark tests done on engine oils these days. But those tests don’t truly replicate the conditions in an engine. There are three stages of lubrication in an engine.

The three types of engine lubrication are hydrodynamic full film, mixed film, and boundary condition. All these stages of lubrication occur at almost the same time in an engine.

The hydrodynamic full film is where the engine bearings live. In the mixed film, there’s a partial oil film and a partial additive film. In the boundary condition, most of the work is done by the additives.

The key to having proper engine lubricant wear testing is having all three stages of lubrication, the right metallurgy, the right speed, loads, and temperatures. All these things that happen in an engine need to be replicated to get the actual results.

But the reality is that there are no devices that can predict what happens in an engine. There are a lot of lube testing devices used to grade and judge the quality of oils.

However, the issue is that none of those devices don’t use real engine parts. The contact and metallurgy used in these devices don’t replicate the reality of an engine. So, it doesn’t tell what happens in the engine.

Engines are too complicated to predict using such bench tests. When you add things like fuel dilution, blow-by, and condensation, it makes things more complicated. Different engines also have different metallurgies going on.

That’s why API ratings are useful. They are given after an engine oil passes several engine tests instead of bench tests. Engine tests are truly the only way to know what an oil is going to do in an engine.

However, there are more types of engines than the ones used to test the API ratings. Some engine oils work well in some cars while others don’t. The difference could be due to the oil itself, the environment, or even the fuel you’re using.

You also have to keep in mind that certain engine types prefer certain oils. For example, older engines like high Zinc oils. There are so many variables at play here. So, that leads to the main question of how you can choose the right oil for your engine.

The chemistry of one engine oil may not be liked by another. So, used oil analysis can help you narrow down what chemistry works well for an engine. All the variables play a role when doing such tests.

You can’t determine which engine oil to use simply by looking at a spec sheet. But you can use them to narrow down the search for the possible candidates. From there, it’ll take a bit of trial and error and testing different engine oils to know which one is ideal for your engine.

Is ZDDP The Only Thing That Matters For More Engine Protection?

After doing a ring-on-liner test on both the classic and regular Castrol GTX oils, their effect on engine wear was measured. The regular Castrol oil had about 80 PPM of Iron in it and the classic oil had about 20 PPM of Iron in it.

So, you can clearly see has a much lower rate of engine wear. So, there’s no doubt that ZDDP matters when it comes to protecting the engine from wearing out. But many people overlook the importance of the right chemistry.

Along with the level of ZDDP, the detergents also matter. There has to be the right balance of Zinc and detergents to make the oil work. When the additives make a good balance, that gives a superior performance.

right oil chemistry

It becomes clear from testing the Valvoline VR1 racing oil. Both the classic oil and the regular VR1 oil have the same level of Zinc. However, the new oil had fewer detergents.

By lowering the number of detergents, the ZDDP worked better in the regular VR1 and gave better wear protection. I have a detailed guide on the ZDDP content of Valvoline VR1 racing oil. Check it out if you’re interested.

So, there are two things you can take away from this discussion. Number one – Zinc absolutely matters and absolutely reduces engine wear. Number two – the combination of the right additives is just as important as using the right amount of Zinc.


Is the regular Castrol GTX oil a good racing oil?

In my opinion, it’s not a good racing oil. The level of detergents and Zinc are both in line with the requirements for passenger cars. While this oil is good for that purpose, it’s not a good choice for a racing vehicle.

When was Castrol GTX oil first introduced?

The oil was first introduced in 1968. So, it’s been around for more than 50 years at this stage with changes in its formulation. That’s why you’d find the classic and regular variant these days.

What is the equation of knowing the stage of lubrication?

The equation used to determine the lubrication stage is Viscosity x Speed/Load. The piston rings go through all lubrication stages in one cycle. You can use this equation to know the lubrication stage at a given moment.

Is the Classic Castrol GTX oil applicable for racing vehicles?

The classic GTX oil has a high amount of ZDDP and detergents. So, it’s a good racing oil. Depending on your vehicle, you should compare it with Valvoline VR1 oil and Mobil 1. Compare them and pick your favorite.


So, does Castrol GTX have zinc? Now, you should know the detailed answer to that question after following this guide. The classic Castrol GTX is really the only Castrol oil worth considering for racing or high-performance applications.

All the other oils are more suited for passenger vehicles. Even the Castrol Edge oils are no different. If you truly want a racing oil, the Valvoline VR1 may be a good choice for you. Check out my guide where I talk about it in detail if you want to know more about it.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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