Is Synthetic Oil Better for Diesel Engines?

Is synthetic oil better for diesel engines? In my humble opinion, the answer would be yes. I will try to present why I feel this is the case. There is abundant real world experience and data to back up that synthetic oils are in fact a better choice for diesel engines.

Conventional vs. Synthetic

When comparing conventional diesel engine oil to synthetic diesel engine oil, the first area that must be looked at would be the molecular structure of both. Conventional lubricants come from the ground and are refined from crude oil. Crude oil naturally contains contaminating elements such as oxygen, sulfur, and nitrogen. In addition to these elements, there are metal components such as vanadium and nickel that are found within crude oil, and unfortunately the refining process cannot remove these unwanted byproducts.

During the refining process of crude oil, the various types of molecules found in the crude oil are separated by weight. This means that the various types of oils that are created from the processing have molecules of similar weight but of different or dissimilar structures. Because petroleum or mineral-based base stock oils have a molecular structure of differing sizes, the performance is uneven and erratic, which can lead to higher than normal fluid friction.

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Synthetic motor oil is chemically engineered so that the base stock oils that are formed are pure oils. They are specifically designed to contain no unwanted contaminants or byproducts and only to have molecules that are useful and engineered for specific purposes. Because of their purity and versatility, the molecular structure of synthetic base stock oils is uniform. Due to the synthetics having a molecular structure, they deliver high performance properties such as reduced friction, superior fuel efficiency, superior film strength, and superior extreme temperature performance.

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

Synthetics naturally offer performance benefits that conventional or regular lubricants cannot obtain or provide. In terms of comparing the molecular structure of conventional oil versus synthetic, their obvious differences clearly display that a synthetic oil is better for diesel engines.

For the professional diesel engine owner that relies on their diesel equipment operating at full efficiency, synthetic oils will help the owner/operator save money and keep the diesel fleet running stronger, better, and for a longer period of time than conventional oils.

Maximum Diesel Engine Protection

The job of a diesel engine oil is to keep metal surfaces in your diesel motor from touching and causing unwanted wear. There is only a very thin microscopic film to keep the metal surfaces separated. A top quality synthetic motor oil technically provides a much tougher fluid film than does conventional oils. The reason synthetics are better is due to their tough and durable base stock oils. They are superior at reducing wear and friction and help to maximize your diesel engine’s service life. Let’s take a look at a specific diesel engine oil manufactured by AMSOIL.

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 15W40 is their premier synthetic diesel engine oil. When a diesel engine experiences a reduction in performance and horse power, inferior fuel economy, a loss of compression, and an increase in oil consumption, then it is safe to say these issues were caused due to abnormal wear on rings and cylinder liners.

AMSOIL chemists had set out to engineer a diesel engine oil that could minimize and hopefully eliminate unnecessary ring wear and cylinder liner wear. To accomplish this, they had to put their more than 40 years of synthetic oil formulation experience to design the finest diesel engine oil on the market. Independent testing has shown that AMSOIL Signature Series Max-Duty Diesel Engine Oil delivers 6 times more wear protection than conventional diesel engine oils.

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

The results were based on independent testing in the Detroit Diesel DD-13 Scuffing Test for specification DFS93K222 using a 5W30 as worst case scenario. By offering 6 times better wear protection, the diesel fleet operator gets an engine oil that delivers the type of security and protection your diesel engine deserves.

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So, is synthetic oil better for diesel engines? Yes, without a doubt, when they can provide 6 times better protection than conventional oils!

Controlling Viscosity

Diesel engine manufacturers specify a specific viscosity for their engines. Extensive tests have been conducted, and at the end of the day, diesel engine engineers have chosen the optimal viscosity grade for that engine. It is for that reason that it is important to choose the correct viscosity and follow the engine manufacturers recommendation. It is also the responsibility of the diesel engine oil manufacturer to develop an engine oil that can withstand the tough demands and difficult operating conditions found in an engine and thus provide an engine oil than can withstand those conditions.

Also, the motor oil manufacturer should provide an engine oil than can maintain its viscosity through the recommended service life as stated in the owners manual. Unfortunately, many conventional and synthetic engine oil manufacturers design and formulate engine oils to meet only minimum standards and at the lowest possible selling price. These types of low priced diesel engine oils are falling way short in terms of providing the necessary protection that a hard work diesel engine deserves.

AMSOIL takes a completely different approach in formulating a diesel engine. Their philosophy is to create the absolute finest synthetic diesel engine oil that can far exceed basic industry standards. Performance is the #1 priority, with price being a distant second. Only the finest synthetic base stock oils are utilized. For example, AMSOIL Signature Series Max-Duty Diesel Engine Oils are naturally resistant to oxidation, plus they are also resistant to the thickening effects of soot contamination that are common in diesel engines.

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

AMSOIL uses the finest and most robust volume of detergent/dispersant additives in their unique additive chemistry. This boosted detergent/dispersant package was created to keep soot particles suspended independently. By keeping soot particles suspended in the oil, this goes a long way in helping to prevent the formation of bigger, wear-causing particles. By keeping soot related wear under control, viscosity increase is minimized.

Because of the high quality synthetic base oils that are utilized and their ability to withstand extremely high temperatures, AMSOIL Signature Series Max-Duty Diesel Engine Oil offers lower volatility. This lower volatility property helps Signature Series maintain its viscosity even after high temperature service. This oil delivers the finest diesel engine protection and efficiency.

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Broad Temperature Range Performance

Conventional diesel engine oils contain wax, which can lead to the oil thickening up in sub-zero temperatures. It has been found that 80% of wear is caused during cold dry starts. When a conventional oil becomes thicker in colder temperatures, this leads to difficulty in starting, reduction in oil flow, and unnecessary wear. On the other hand, AMSOIL Signature Series Max-Duty Synthetic Diesel Oil contains no wax, thus it flows freely and quickly during cold temperatures. This translates to easier starting, reduced wear, and improved and quick oil flow to critical engine parts.

When temperatures inside an engine become hot, conventional oils are less thermally stable, which means they start to break down when exposed to heat. Thermal breakdown leads to oxidation, which leads to sludge and other deposits from forming in an engine. As stated before, mineral or petroleum base stock oils contain unwanted byproducts or contaminants, which exasperates the accumulation of deposits when the conventional oil breaks down due to heat.

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

On the other hand, quality synthetic base oils as used in the formulation of AMSOIL Signature Series Max-Duty Diesel Engine Oil resist thermal or heat breakdown far better than conventional oils and also competitive synthetic diesel oils.

Less Oil Consumption

Synthetic diesel engine oils such as AMSOIL’s Signature Series are engineered with a uniform molecular structure. These base oils are thermally stable and due to their molecular structure, they minimum oil evaporation and keep the lubricant¬† doing what it was intended to do: protecting your diesel engine. Burn off or volatility happens when an oil is exposed to extremely high temperatures and gets hot. This is especially true in conventional mineral or petroleum based oils where their uneven molecular structure when exposed to heat causes the lighter molecules to evaporate or burn off.

AMSOIL Signature Series Synthetic Diesel Engine Oil falls far below the API limit for volatility. This translates to less oil consumption, vastly reducing the need for frequent engine oil top offs and from an environmental point of view, vastly reduced vehicle emissions. As an example on how AMSOIL Signature Series Max-Duty delivers much less oil consumption, it was tested in the Caterpillar- 1N Oil Consumption Test. The final results after the test was conducted showed that AMSOIL Signature Series delivered up to 76% less oil consumption than is required by the American Petroleum Institute CK-4 standard.

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

Its low rate of burn off or volatility vastly reduces oil consumption during engine operation, and thus will pass less engine oil vapor into the combustion chamber. It’s a win-win situation, because as a fleet owner, you will waste less engine oil and also your vehicle/diesel equipment will emit less harmful emissions into the environment. At the end of the day, less consumption means more dollars in your pocket.

Oil Compatibility

AMSOIL Signature Series Max-Duty Synthetic Diesel Oil is compatible with other synthetic motor oils and other conventional oils. AMSOIL’s synthetic diesel oils can be mixed with other engine oils, but understand that when mixing AMSOIL’s synthetic engine oils with other engine oils, this will shorten the oil’s life expectancy and will reduce the benefits and performance that AMSOIL’s synthetic engine oils were engineered to deliver. Also, please note that AMSOIL is a high performance engine oil that was formulated to provide extended drain intervals. If the need arises where AMSOIL’s synthetic diesel oils are mixed with other oils such as conventional diesel oils, AMSOIL will not support extended drain intervals when two different oils have been mixed.

Also, AMSOIL does not recommend that any aftermarket oil additives are used with their synthetic diesel oils.

Oil Service Life

Diesel engine oil service life for heavy duty on and off road equipment is 3 times the original equipment manufacturers recommendation, but is not to exceed 60,000 miles or 600 hours or 1 year, whichever comes first. For turbo diesel pickups, the recommended diesel oil service life is 2 times the original equipment manufacturers recommendation, though not to exceed 25,000 miles or 1 year, whichever comes first. For competition diesel service life, AMSOIL Signature Series can provide superior protection for multiple events, but it is highly recommended that the oil be put through scientific oil analysis so as to determine the appropriate drain intervals.


At this point, let’s take stock of what has been presented in terms of facts and information to help answer the question: is synthetic oil better for diesel engines? Independent tests have supported the claim that synthetic oils are better for diesel engines because AMSOIL’s Signature Series Max-Duty Diesel Oil delivers 6 times better wear protection. Signature Series also maintains and controls viscosity far better than conventional oils. We also showed how synthetic oils provide better all around protection over a broader range of temperatures, whether cold or hot, far better than conventional oils.

In terms of minimizing oil consumption, an independent test proved that AMSOIL Signature Series provide 76% less oil consumption than was required by API CK-4 standards. Additionally AMSOIL Signature Series Synthetic Diesel Oil provides much longer drain intervals than conventional and competitive synthetic oils. So, in conclusion that yes, synthetic oils are a far better choice for diesel engines and they deliver incredible benefits to the diesel fleet owner in terms of longer equipment life, better fuel economy, less oil consumption, less downtime, and at the end of the day, help the bottom line of any professional diesel equipment operation.