Best Synthetic Engine Oil for Diesel Engines

You’ve invested a lot of money into your fleet of equipment. You earn money when your diesel equipment is either in the field or on the road. There’s no money to be earned when your equipment is in the shop getting repaired or being maintained. In a perfect world, it would be great to have your machinery working 24/7 earning you money. Realistically, there is a simple strategy to give you the best opportunity to minimize downtime and reduce maintenance costs by using the best synthetic engine oil for diesel engines.

The 1st and probably the most important choice is the type of engine oil you choose to protect your diesel engines. We propose in this blog that you choose the best synthetic engine oil for for diesel engines. The field is very crowded and choosing the right oil is not so simple, so we’ll try to present information that will give you a better understanding and a clearer picture on what is the best engine oil for diesel engines.

Once you’ve made that decision, regardless of the investment required to purchase that oil, we’ll present data and most importantly, how you can benefit from such a choice.

What is the job of an engine oil? A good quality motor oil is there to protect critical engine parts. It must cool the engine, provide a protective film so as to reduce friction and unnecessary wear, and for a diesel engine an oil needs to neutralize harmful acids that are formed and are present. It must hopefully protect, last, and do its job over an extended period of time. The oil should maintain engine cleanliness, be able to withstand high temperatures, and not burn off or evaporate. Critically, during cold dry start ups, such an oil must flow quickly to engine parts so as to protect them from wear.

Please note that during cold dry start ups, 80% of engine wear occurs at this critical time. Knowing this quickly narrows your choices because only a small number of engine oils can effectively protect your engine during cold dry starts.

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

Not All Base Oil Stocks Are the Same

Engine oil manufacturers or blenders have choices when engineering diesel motor oils. They may decide to design a diesel engine oil that meets minimum industry standards and goes to market at a very low price. They can choose to manufacture an oil that far exceeds industry standards and delivers untold benefits and performance without regard to the costs of the ingredients for such a formula.

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If you want to protect the large investment in equipment and keep that equipment out of the shop and working earning you money, it is important to very carefully consider what type of oil you will ultimately make part of your program. It is highly doubtful that a diesel engine oil that barely meets minimum specifications and sells at the lowest price could provide the type of protection needed to make a hard working diesel engine perform over a long service life.

To better understand why there are differences between both a low price oil versus a high performance oil that initially requires a much higher investment but ultimately is the lowest cost engine oil in the long run, we need to look at the ingredients in more depth.

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

Base stock oils are in essence the architectural foundation of an engine oil, or for that matter any lubricant. The choice the manufacturer makes in the base oil they will utilize will determine the type of performance and benefits that oil will deliver. In other words, the base oil is the critical ingredient for any lubricant. There are two choices for base oils. There are mineral or petroleum base stock oils or there are man-made synthetic base stock oils.

The first major difference between a mineral or petroleum base stock oil versus synthetic is their molecular structure. The molecular structure of a mineral based oil is uneven and is composed of different sized molecules. Because of their different sized molecules, they inherently flow unevenly. This uneven flowability immediately delivers an unwanted effect, which is fluid friction.

Also, the uneven sized molecules tend to be thick and flow with too much resistance or fluid friction during cold dry starts. They take too long to flow to critical engine parts, leaving them unprotected and causing unnecessary wear.

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Any sort of friction in an engine will ultimately end up causing unwanted heat and other negative issues down the road. When heat is present in a lubricant, temperatures rise and the potential for component wear increases.

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

Understand that when choosing a mineral base oil, there is only so much refining that can be done. The refining process is conducted so as to remove unwanted byproducts. These byproducts, if left in an engine oil, will ultimately cause sludge, varnish, and other unwanted deposits.

Yes, there are accounting reasons in choosing petroleum or mineral base stocks when manufacturing an engine oil for diesels. These low price oils do an adequate job based on their cost. There are compromises to consider. For example, mineral oils deliver poor performance when weather is cold due to the wax content in the base stocks. When encountering high temperature operating conditions in an engine, their average thermal stability rating means that there is a good chance of some of the oil burning away and evaporating, which leads to oil consumption and sludge formation within the engine.

For light duty operations, petroleum based or conventional diesel oils are sufficient and for that application the low cost is justified.

For the heavy duty fleet operator whose equipment is predominately operating under more difficult long-term usage, these petroleum or conventional base stock oils are insufficient. True, the low cost is tempting and unfortunately, in many cases these light duty oils seem to be the norm.

For the open minded thinker who is looking to protect their investment and maximize profits, there is a clear alternative, and that alternative is synthetic base stock oils.

The first glaring technical difference that synthetics offer over mineral base oils lies in their molecular structure. These man-made base stocks have a perfectly even sized and consistent molecular structure. Due to these same sized molecules, they offer little to no fluid friction. This is extremely important for a diesel engine when this engine is asked to start after it’s been sitting for some time.

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

As we stated earlier, the fact is that 80% of all engine wear occurs during cold dry starts. Synthetic oils can virtually eliminate this unwanted and unnecessary wear. They flow quickly and easily to critical engine components. In fact, synthetics are what is known as polar. They tend to cling and stick to engine components. When a motor is shut down, what is left behind is a polar film of synthetic oil, which means that even after sitting for days on end, the engine still has a film of synthetic oil due to its polarity or affinity to metal surfaces.

Synthetic base stock oils also differ from mineral base oils in that they’re man-made and have no inherent unwanted byproducts, like conventional mineral base stocks do. Another area where synthetics are superior is in their ability to withstand much higher temperatures. They offer far superior thermal stability. In terms of oil consumption, synthetic oils will offer very little oil consumption when compared to regular petroleum base oils. They offer fantastic flowability characteristics during extremely cold temperatures, far below mineral based oils.

Now that we’ve reached the conclusion that synthetic base oils far outperform conventional base oils, we will continue to analyze what is the best synthetic engine oil for diesel engines.

Diesel Additives for Extreme Performance

Diesel engines provide a tough challenge for lubricant chemists and formulators. There are many areas that need to be considered when formulating a diesel engine oil that can effectively protect hard working engine components over a broad range of working environments. The first issue that must be dealt with in a diesel engine will be how to effectively neutralize the corrosive acids that form when an engine is in operation.

To fight acids in a diesel engine, an oil manufacturer utilizes detergent/dispersant chemistry. When looking at a technical data sheet of any diesel motor oil, you will notice what is called Total Base Number, or TBN. This value gives you a hint at an oil’s ability at neutralizing acids. Most conventional lower priced diesel engine oils offer a TBN value from 5 to 8. A quality synthetic diesel motor oil should offer a TBN value of 10+. The higher the number, in theory, the better the oil is at neutralizing corrosive acids in an engine.

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Once that number is determined, the next agenda for chemists is how to make this acid fighting chemistry last and provide the acid neutralizing protection for a long duration of service life. TBN retention is critical. The longer that TBN number stays the same, the longer it can effectively battle acid. Conventional mineral based diesel engine oils that sell for a lower price do not offer the type of TBN retention necessary for a hardworking diesel engine. If an oil is unable to safely and effectively neutralize acids, then expect a negative vicious cycle of oil degradation to quickly transpire.

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

Once a diesel engine stops neutralizing acid, severe engine wear and oil degradation happens quickly. If this degraded oil is left in the engine and unchanged, then it is possible for catastrophic failure to occur. If an owner of a diesel fleet is set on continuing with lower price mineral oil, then it is imperative that the oil be change frequently.

For those that would like to learn about an engine that offers extremely robust and long lasting TBN retention, then consider a 100% pure synthetic heavy duty diesel motor oil such as one manufactured by AMSOIL. AMSOIL synthetic diesel oils have been tested independently, and the results show their diesel oils to provide TBN retention that far exceeds industry standards.

For an oil to offer superior TBN retention requires the use of only top tier detergents and dispersants. Initially these types of additives cost more but long term they last far, far longer and are capable of providing incredible TBN retention and most importantly, they easily are able to neutralize acids. They are designed specifically for heavy duty hard working diesel engines. These types of synthetics are truly high performance.

‘The fact is, only these types of oils should be considered for any heavy duty diesel equipment. Why spend hundreds of thousands of dollars on equipment only to decide to use minimal standard conventional diesel oils? Match lubricant quality to the quality of your equipment, and your equipment will far outlast those that continue to think cheap oils can get the job done. There’s no logic in that kind of thinking.

The next area we must address is friction. Friction needs to be minimized as much as possible if an engine stands a chance of offering a long service life. If there’s a way to protect parts from wearing away, it is important to investigate how one can minimize friction.

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

As we stated earlier when discussing the difference in base stocks, it is known that mineral base oils have different sized molecules. These uneven molecule sizes create fluid friction. By deciding to use mineral oils, you’ve already inherited from day one unwanted friction. In other words, you’re starting out with a minus.

There are ways that lubricant manufacturers have overcome this fluid friction, but at the end of the day, why not choose an oil that reduce fluid friction right from the get go? You know, like synthetics! With synthetics, you start on the positive side of the equation, not the negative. Synthetic base oils naturally reduce friction.

To enhance a synthetic’s natural ability to reduce friction, specialized friction modifiers are added as an enhancement.

Chemists experienced in formulating high performance synthetic diesel engine oils only choose the finest friction modifiers. These unique and interesting additives have a very strong attraction or affinity to metal surfaces. By incorporating these types of friction modifiers into a synthetic diesel oil, engine components will benefit greatly.

Beyond the friction reducing abilities of the base oil, friction modifiers further reduce friction, which leads to lower temperatures, less metal wear, and longer engine and oil life. By conducting oil analysis, once can see proof of this friction reduction when utilizing a quality synthetic engine oil. Oil analysis will show less metal wear.

Another beneficial byproduct when reducing friction in an engine through the use of a quality synthetic diesel engine oil will be an improvement in fuel economy. Improved fuel economy is possible for diesel fleet owners because synthetic oils reduce friction, thereby allowing an engine to work more efficiently, thus consuming less fuel. Also, when friction is reduced, an engine can and will produce extra power.

The next additives utilized in a quality synthetic engine oil for diesels would be anti-wear agents. Anti-wear agents chemically react with metal surfaces to protect them from close contact with other metal surfaces. Anti-wear additives work synergistically with friction modifiers to reduce premature wear and also help withstand extreme pressures.

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

Anti-wear agents are there to prevent metal-to-metal contact.

The potential for moisture forming in a diesel engine are high. To guard and protect engine parts from moisture, rust and corrosion inhibitors are added. Rust and corrosion inhibitors create a chemical bond onto the metal surfaces to protect them from moisture and acids. This chemical bond does not allow any moisture or acids to penetrate and stops any attack to the metal surfaces.

Another area that must be addressed in a diesel engine is foaming. Foam acts as an insulator. It can trap heat that exists when an engine is in full operation. If foam is allowed to build inside a motor, temperatures can skyrocket. Lubricant engineers typically add anti-foam additives to help stop and minimize any foaming. A quality anti-foam additive in the correct quantity can reduce operating temperatures anywhere from 20 to 50 degrees F. This is possible because the anti-foam additive is dispersing the foam, thus releasing the trapped heat.

Another interesting additive that is incorporated into a quality synthetic diesel oil would be an oiliness additive. This interesting additive allows the oil to literally penetrate the subsurface of the metal, thus allowing for superior and better lubrication.

The final but very critical additive used in a multi viscosity diesel engine oil is called a viscosity index improver additive. This additive is like a coiled spring. When the temperature is cold, the spring is compressed. When the temperature is high, the coil expands. Inside an engine, there is a huge amount of shearing action. Think of shearing like a fast moving scissors, opening and closing very quickly.

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

Lower priced conventional diesel oils utilize low performing, low cost viscosity index improver additives. These VI improvers tend to shear quickly. The coils of the VI improver are easily cut by the imaginary scissors we discussed.

When an oil loses its viscosity, it is due to the viscosity index improver additive breaking down. In other words, a 15W40 oil shears down to 15W30, and then is goes to 15W20, etc. Once the shearing happens, the oil will not reach its specified 40 weight. A quality heavy duty synthetic diesel engine oil will utilize shear stable viscosity index improver additives. They are very resistant to this shearing action and they can retain the proper viscosity over a long drain period.

For example, AMSOIL Signature Series Diesel Engine Oil is extremely shear stable and after independent testing, it exhibits very little viscosity loss. Its superior shear stability was proven when it was tested using the ASTM D-5481 High Temperature/High Shear Viscosity Test. It far outscored all conventional diesel oils.

The final additive that contributes to a superior diesel engine oil would be the use of antioxidant agents. We already know that synthetic base oils withstand high temperatures and are thermally stable. They withstand tremendous heat and resist oxidation. Adding an antioxidant agent to the formula further enhances a diesel engine oil’s ability to resist oxidation. These oxidation inhibitors will help prevent the formation of damaging deposits such as sludge, carbon, and varnish.

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

Mineral based diesel oils are more susceptible to oxidation. If oxidation occurs in a diesel engine, deposits start to appear and the oil starts to thicken, leading to a reduction in oil life and unnecessary engine wear.

By choosing the best synthetic engine oil for diesel engines formulated with the finest oxidation inhibitors, your motor is getting the 1-2 punch protection it needs to realize long drain intervals and long engine service life.

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

Finally, What is the Best Synthetic Engine Oil for Diesel Engines?

What is the best choice for your diesel fleet? We’ve discussed many things when comparing mineral based oils versus synthetic based oils. We’ve tried to present a different perspective or philosophical concept in the hopes of making your choice more informed. It can be difficult to disrupt the status quo. Still, it goes without saying that if you’re running a fleet of diesel equipment and your livelihood depends on that equipment staying out of the shop and out in the road or at the construction site earning money, then why not take a strong look at an alternative choice.

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

If that alternative can in fact deliver more benefits to your operation, then it’s worth the time to consider it. Ask yourself this question. What is more expensive: having to take an engine out of the field and into the shop for a rebuild before its time? Or deciding to invest in the absolute best synthetic engine oil for diesel engines? Is that engine rebuild worth using a cheaper priced oil versus having to invest in a higher priced synthetic engine oil that gives you 2 to 3 times longer engine life? At the end of the day, which is providing you the best value or the best return on your investment?

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

AMSOIL synthetic diesel engine oils time and again have proven to be the most value added lubricant on the market. For over 45 years as the leader in synthetic lubricants, AMSOIL has offered the best synthetic engine oil for diesel engines.