Best Engine Oil for Diesels

Whenever there is the debate on the best engine oil for diesels, there needs to be a discussion about certain critical factors on what constitutes the makeup of a superior oil before a decision can be made. Meeting EPA standards and dealing with the ever changing quality of diesel fuel has forced diesel manufacturers to design and develop complex computerized fuel deliver systems and high tech exhaust systems.

The need has arisen for diesel engine oil manufacturers to keep pace with this advanced technology and offer the type of protection diesel engines are crying for.

Engine oil formulators must decide which philosophical road they will follow in terms of the quality and performance they intend to pursue. Will they design a conventional diesel oil that meets minimum industry standards and also meets a low price point? Will they follow a totally different philosophical approach by formulating a state-of-the-art synthetic motor oil that exceeds industry standards and delivers to diesel engines superior protection and performance?

In this blog post, we’ll attempt to compare these two philosophical approaches in engine oil blending. By offering differences between a standard diesel motor oil vs. a top tier full synthetic diesel oil, you’ll have more facts to help you reach a decision on the best engine oil for diesels.

Components of a Diesel Motor Oil

When trying to decide the best engine oil for diesels, the first factor to consider is what base oil stocks are being chosen by the engine oil manufacturer. Choosing the correct base oil stocks essentially determines the type of foundation the engine oil will be built from.

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Like all quality architecture, having the strongest and most solid foundation is the essence of quality performance and longevity, thus deciding on the highest quality and best base oil stocks logically will be a very important determining factor in the quality of the diesel engine oil.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 15W-40.
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 15W-40

To reach these goals, the formulator must choose ingredients such as detergent/dispersants, friction modifiers, anti-wear agents, rust and corrosion inhibitors, oiliness additives, and VI improvers that are shear stable if needed. Understanding how all of these components work together and the type of performance they deliver will illustrate whether the diesel oil will offer the necessary protection when exposed to severe operating conditions typically found in modern diesel engines.

Base Stock Oils: Which Are Better?

Over the last ten years, motor designs and base oils have gone through radical changes. The operating environments found in modern diesel engines are severe, thus forcing motor oil manufacturers to try and come up with formulas that can withstand a broad range of severe operating temperatures. It is commonplace now for engines to operate at much higher temperatures than in the past, causing tremendous demands on engine oils.

Base stocks from the past are simply not up to the task of withstanding these very high thermal conditions. These high temperatures can cause oils to thin down, oxidize, and burn away.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 5W-40.
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 5W-40

On the opposite side of the temperature spectrum, it is critical for a diesel engine oil to provide immediate lubrication during dry cold starts. If a motor oil does not flow easily during cold dry starts, critical engine components are left without a crucial film of oil, which leads to unnecessary component wear. Studies have proven that 80% of engine wear occurs during these dry cold starts. Again, this underlines the importance of choosing the correct base stock oils.

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Conventional diesel oils typically are formulated with mineral or petroleum base stock oils. These mineral oils go through a basic refining process just enough so they can provide standard lubricating qualities to a diesel engine. In terms of low temperature protection and high temperature performance, they rate as average or mid-range.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 0W-40.
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 0W-40

Expect adequate thermal or high temperature stability for only light duty service. If such a diesel oil is called on to perform over a long period of time under difficult operating conditions such as high heat, a conventional mineral based diesel oil will start to oxidize and break down. This slow process of oxidizing brings about negative issues, such as potentially harmful engine deposits in the form of sludge, varnish, and gum. These lower priced conventional diesel motor oils tend to consume oil at a faster rate because of their lower levels of thermal stability.

When a manufacturer decides to engineer an oil that meets a lower price point, plus minimum industry standards, the fact is that lower quality base stocks are chosen. These low cost, lower quality base oil stocks have not gone through all the necessary refining processes. By failing to refine out many unwanted byproducts naturally found in crude oil, these byproducts ultimately end up in your motor, causing undue wear and denying the motor a long and profitable service life.

Not only do they consume oil faster, but to maintain the integrity of the engine components, more frequent oil and filter changes are definitely required.

Let’s now discuss the opposite side of the base oil stock spectrum, which is pure synthetic base oils. After we’ve analyzed synthetic oils, you might get a hint as to what is the best engine oil for diesels.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 5W-30.
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 5W-30

The first and probably the most significant physical difference between a mineral or petroleum base stock oil versus a pure synthetic base stock oil lies in the molecular structure of each oil. Mineral based oils naturally have an uneven different sized molecular structure. In simplistic terms, because of the different and uneven sized molecules, they flow with more resistance.

Resistance causes friction, and thus generates more heat. Generally speaking, they flow uneasily in cold temperatures and tend to thin down when temperatures become hot.

Conversely, a pure synthetic base oil has gone through many complex processes to the point where the final product offers perfectly consistent and even molecular structure. The molecules are all of the same size. They flow easily in cold temperatures with minimal or no fluid friction. They withstand extremely high temperatures and are quite thermal stable.

For this reason, there is no doubt from a technical standpoint that a pure synthetic base oil is far superior to a conventional petroleum base stock. A synthetic base stock offers the ultimate foundation when building and formulating a truly high performance diesel engine oil.

Pure synthetic base oils are void of unwanted byproducts. They offer consistent and long service life at elevated temperatures. In terms of minimizing and eliminating unwanted wear during dry cold starts(remember, 80% of engine wear occurs during dry cold starts), the base stock of choice has to be an engine oil formulated with pure synthetic base oils. Not only do they immediately flow quickly to critical engine parts, but interestingly they tend to cling and stay on engine parts after a motor is shut off. You could say synthetic oils are polar and have a strong affinity to metal.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 10W-30.
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 10W-30

Another key factor in the superiority of pure synthetic base oils over mineral base oils is their natural thermal stability. Having a high thermal stability translates to the oil not being consumed unnecessarily when exposed to the  hot operating conditions found in modern diesel engines. The low volatility properties of pure synthetic oils means less oil consumption, less engine deposits, longer component life, and minimal downtime. Maintaining the proper volume of oil in an engine is so critical for the fleet owner, it goes without saying that choosing the best engine oil for diesels is paramount to maximizing profits.

Consuming less oil means less inventory requirements, less maintenance costs, and longer drain intervals. Keeping equipment operating in the field and on the road is the main objective.

AMSOIL Heavy-Duty Synthetic Diesel Oil 15W-40.
AMSOIL Heavy-Duty Synthetic Diesel Oil 15W-40

Naturally, the subject of cost will eventually appear in your mind. On the surface, there seems to be a big difference between the price of a conventional diesel oil versus a pure synthetic diesel oil. If a decision is made purely on price alone without considering other factors, then on a superficial level, deciding on a low price oil seems to be correct. Yet in almost all instances, real world experience and independent data consistently illustrates that choosing the higher quality synthetic diesel oil, though initially a higher investment, will ultimately deliver the most value.

By choosing the best synthetic engine oil for diesel engines, regardless of the cost of the formula, such an oil has proven to deliver the best performance and protection at the lowest cos in the long run. When protecting your massive investment in heavy equipment, the lowest cost insurance would be utilizing the best synthetic oil for diesel engines.

Let us agree the best engine oil for diesel engines needs to be manufactured with synthetic base oil stocks. They provide longer drain intervals, offer superior cold temperature flowability that lowers startup wear, deliver improved high temperature performance and thermal stability which virtually eliminates oxidation, operate over a broad temperature range, and provide consistent viscosity.

AMSOIL Heavy-Duty Synthetic Diesel Oil 5W-40.
AMSOIL Heavy-Duty Synthetic Diesel Oil 5W-40

Detergent/Dispersant Package (TBN) Retention

Total Base Number (TBN) is an important factor that illustrates the effectiveness of an engine oil in neutralizing against corrosive and damaging acids that are continuously produced inside a diesel motor. Many lower priced mineral based diesel engine oils have TBN values between 5 to 7. A superior synthetic diesel motor oil should provide a TBN value of 10+.

This higher number versus lower quality mineral based oils is the first difference in the effectiveness of the diesel oil in neutralizing corrosive and damaging acids. A more important factor is how long can the oil retain its TBN value. Logically, the longer an engine oil retains its original TBN value, the better it can neutralize damaging acids.

AMSOIL Heavy-Duty Synthetic Diesel Oil 10W-30.
AMSOIL Heavy-Duty Synthetic Diesel Oil 10W-30

Standard mineral based diesel oils that have lower TBN values will typically provide much lower TBN retention. It is imperative when using lower quality diesel oils that oil samples are taken frequently and sent off for oil analysis. If such lower quality oils are not scientifically monitored through oil analysis, then potentially catastrophic damage can occur inside the motor if these corrosive acids are not consistently neutralized in the diesel engine oil.

We must always be familiar with an oil engine manufacturer’s philosophy in terms of how they create and design their diesel engine oils. When an engine oil blender offers a low price, the red flag should go up, since it is virtually impossible to create a low price diesel engine oil that offers long lasting TBN retention. As the saying goes, “you get what you pay for.” If the goal is to give your engine the best possible protection against corrosive acids, then a switch in perspective needs to take place. One must now consider only the best engine oil for diesels.

For example, one of the premier pure synthetic oil manufacturers in the United States is AMSOIL. AMSOIL offers only state-of-the-art synthetic oil technology. Time and again, independent tests show their synthetic diesel oils to provide TBN retention results that far exceed industry standards.

Their philosophy has always been to choose only the highest performing additives, without regard to initial cost. In terms of TBN values and retention, they use the absolute finest detergent additive. A high performance detergent additive helps to keep engine components clean and neutralizes acids during combustion.

AMSOIL European Car Formula 5W-30 Improved ESP Synthetic Motor Oil.
AMSOIL European Car Formula 5W-30 Improved ESP Synthetic Motor Oil

Next, only the finest dispersant additives are blended in. These dispersants are there to effectively keep solid contaminants that are found within the oil in colloidal suspension. This results in the prevention of sludge and varnish deposits forming on engine parts. To further enhance TBN retention, AMSOIL goes beyond the industry norm by adding a TBN booster/stabilizer.

This extra step improves the oil’s ability to further neutralize harmful corrosives and acids. This also further extends the engine oil’s service life and drain interval.

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AMSOIL is an example of having a high performance only philosophical approach to manufacturing the best possible oil for equipment. That philosophy has earned AMSOIL an enviable reputation inside the industry as a manufacturer that delivers lubricants that work and provide real benefits.

AMSOIL European Car Formula 5W-40 Improved ESP Synthetic Motor Oil.
AMSOIL European Car Formula 5W-40 Improved ESP Synthetic Motor Oil

How to Eliminate Friction and Wear

As stated earlier, mineral based diesel engine oils utilize base stock oils that naturally come with an uneven molecular structure. This type of uneven molecular structure inherently provides higher levels of fluid friction, so if your goal is to reduce friction and wear inside your diesel motor, then right at the beginning you’re at a severe disadvantage.

Knowing this unfortunate fact of too much natural fluid friction is before we’ve even discussed friction caused by moving parts.

AMSOIL European Car Formula 5W-40 Classic ESP Synthetic Motor Oil.
AMSOIL European Car Formula 5W-40 Classic ESP Synthetic Motor Oil

Conventional diesel motor oils do contain some low cost friction modifier additives, but their effectiveness can only be seen in light duty service. To see real gains in the reduction of friction and wear inside your motor through scientific oil analysis, then one needs to shift thinking only towards the finest engineered synthetic engine oils for diesel motors.

We’ve already learned that man-made synthetic base stock oils offer much lower fluid friction due to their consistent and even sized molecular structure. Because of the complex refining processes these synthetic oils have gone through, they are completely without unwanted byproducts.

These pure base oils flow effortlessly without fluid friction. Right from the get go, the battle against friction and wear is being won through the use of high quality, high performing synthetic base oils.

AMSOIL European Car Formula 0W-40 Classic ESP Synthetic Motor Oil.
AMSOIL European Car Formula 0W-40 Classic ESP Synthetic Motor Oil

Blended into these oils, you will most likely learn about the choice of friction modifiers utilized by chemists. These friction modifiers have an affinity to metal surfaces. When a heavy duty diesel engine is lubricated with such a synthetic oil, these heavy duty friction modifiers will radically reduce friction. By reducing friction, lower temperatures are realized and less wear metals are seen during oil analysis.

A top-of-the-line friction modifier will offer the diesel fleet owner improved fuel economy and an increase of engine efficiency and power. Further, a top tier synthetic diesel engine oil such as produced by AMSOIL will contain heavy duty anti-wear agents. These unique additives will provide a reduction in friction and wear and they also help prevent metal to metal contact.

Friction modifiers and anti-wear agents will typically work synergistically to reduce or eliminate friction and premature wear. Together, along with the synthetic base oils, they will enhance fuel economy through the reduction in friction and drag. Also, an added benefit is their ability together to withstand extreme pressure.

AMSOIL SAE 15W-40 Heavy-Duty Diesel and Marine Oil.
AMSOIL SAE 15W-40 Heavy-Duty Diesel and Marine Oil

Still More Additives Needed!

Modern diesel engines provide a brutal environment that seemingly is impossible to deal with. Today’s diesel fleet owner needs to stay open minded and consider the reality of what is truly required to effectively protect his substantial investment in diesel equipment. The notion that oil is just oil simply will not stand the test of time today.

Not only is it critical that the best motor oil for diesel engines utilizes the finest base stock oils available, but also understanding its complex chemistry is equally important. There are other necessary additives that contribute to the best engine oil for diesels.

AMSOIL Series 3000 5W-30 Synthetic Heavy Duty Diesel Oil.
AMSOIL Series 3000 5W-30 Synthetic Heavy Duty Diesel Oil

Before we go any further, let’s face facts. When comparing conventional or mineral based diesel engine oils that are selling at a low price versus high performance pure synthetic diesel engine oils that carries a substantially higher price, one must conclude that there are real differences in the quality of ingredients and the volume of those ingredients being used.

Top tier diesel engine oils whose philosophy is performance first and price second are typically using robust volumes of the finest additives available. At the end of the day, such a product can offer huge benefits to the end user.

AMSOIL Premium Protection 20W-50 Synthetic Motor Oil.
AMSOIL Premium Protection 20W-50 Synthetic Motor Oil

Oxidation inhibitors are important in helping to prevent carbon deposits, varnish, sludge, and other deposits from forming due to oxidation.  Heavy duty oxidation inhibitors help to reduce engine deposits and oil thickening, which extend oil life. Starting out with a pure synthetic oil as the foundation, along with oxidation inhibitors, gives the oil a one-two punch in offering real long drain intervals.

To protect engine components from damaging acids and moisture, an engine oil requires rust and corrosion inhibitors. Rust and corrosion inhibitors create a chemical bond on the metal surface which prevents acid and moisture from attacking and penetrating the metal surfaces.

AMSOIL Premium Protection 10W-40 Synthetic Motor Oil.
AMSOIL Premium Protection 10W-40 Synthetic Motor Oil

When doing research on what is the best engine oil for diesels, looking at the types of additives used by the chemist will give a good indication of how the manufacturer perceives performance and providing real value to the customer. One interesting additive that is just another indicator to diesel engine quality would be it containing an oiliness additive.

An oiliness additive gives an oil the ability to penetrate the surface of metal, thus providing superior lubrication. Like all additives, there is low quality and high quality. A pioneer like AMSOIL will deliver a synthetic diesel oil with a lubricant that effectively penetrates metal surfaces and provides the kind of heavy duty protection against wear.

AMSOIL DOMINATOR 20W-50 Competition Diesel Oil.
AMSOIL DOMINATOR® 20W-50 Competition Diesel Oil

Another critical area that illustrates an oil’s superiority would be its high temperature / high shear viscosity. This is one area which can clearly separate a motor oil manufacturer from another. A quick way to determine an oil’s ability to maintain its viscosity over a long period of time is to check whether the oil was tested using the ASTM D-5481 high temperature / high shear viscosity test.

The lower the number, the more shear stable or stay-in-grade rating the diesel oil will have. Most conventional oils and even so-called “premium” motor oils score in the high teens and twenties.


For example, AMSOIL’s 15W40 Pure Synthetic Diesel Engine Oil scored a 4.5 on this test. This means the oil will stay in grade over an extended drain interval. By using a shear stable viscosity index improver, an oil will thin out less at high temperature and thicken less under low temperature. This is a vitally important point when considering what is the best diesel engine oil in the market.


So, What is the Best Engine Oil for Diesels?


A lot of information and concepts have been provided in this in-depth discussion on the best engine oil for diesel trucks and equipment. Sometimes it’s hard to break away from a certain mindset in regards to pricing of lubricants, specifically engine oils. Consider this: think carefully how much money is invested in your diesel vehicle, your fleet, or whatever diesel equipment you are running. Then ask yourself, what is cheaper?

An engine rebuild and the loss of revenues because of the engine being in the shop, or the use of the best synthetic oil for diesel engines? Hopefully the answer you reach will be one that allows your motor to have a long life and help you earn the profits you deserve. In some way, we’d like to think this article has helped you make the right decision.