Straight 40 Weight Motor Oil vs 15w40 Synthetic Diesel Oil: Which Is Better ?

For years, you may have run and continue to run a straight 40 weight motor oil in your diesel engine. Also, the thought may have occurred to you whether switching to a multi-viscosity 15W40 synthetic diesel oil might be a better option. To determine if a straight weight engine oil is the best for your diesel engine, first, we need to understand how it differs from multi-viscosity engine oil.

The engine oil’s viscosity is typically printed on the motor oil label for everyone to see. The thing is, most people don’t know what those numbers (e.g. 15W40) mean. For this post’s discussion, we’ll basically say that the majority of engine oils sold today are multi-viscosity. The first number you see listed on the label refers to the engine oil’s viscosity, or its resistance to flow when cold (e.g. 15W.) The “W” after the first number means “winter.” The lower the number, the easier the engine oil will flow when it’s cold. This means a 5W50 motor oil will flow easier and quicker at start-up than a 20W50 motor oil.

Did you know that 80% of engine wear occurs during dry and cold starts? This means that every morning when you start up your diesel engine, the potential of your engine experiencing 80% of its wear will occur during this period unless you’re using an engine oil that will flow fast and easy during these dry and cold starts. So, ask yourself this question: are you confident that the straight 40 weight motor oil you’ve been running does in fact provide your diesel engine with immediate protection during cold dry starts?

It’s very easy to adopt the “out of sight, out of mind” type of thinking. In the case of commercial diesel engines, we’ll try as best as we can to present ideas and concepts in this blog post that can help you maximize your engine’s performance and protection. Understand that there are simple strategies in improving your diesel engine’s performance and protection.

Understanding Cold/Dry Starts

The most important point to consider is the better an engine oil can flow during cold temperatures, the better it can provide dry/cold start-up protection. As we’ve said before, most engine wear happens during initial start-up when the diesel engine is cold and has been idle long enough so that the oil has dripped back completely into the oil sump, which occurs overnight. Obviously, the best engine wear protection will happen with engine oil that will flow fast and easy when you turn the ignition key on.

Engine oil that may have too thick of a viscosity may not flow readily and quickly through the small engine oil passages, for example, the main bearings or the top part of the engine. This will cause the motor to run with little oil pressure for a few critical seconds.

Honestly speaking, if we focus purely on cold dry starts. a straight 40 weight motor oil is not the ideal lubricant that will deliver minimal wear during the initial start-up of a diesel engine. As you’ll see later in the post, there are far superior motor oil choices that you can utilize which will bring you and your diesel engine huge benefits.

Normal Engine Operating Temperatures

The second number seen in multi-vis engine oil (e.g. 40) designates the oil’s viscosity at 100 degrees centigrade (212 degrees Fahrenheit), which identifies the normal operating temperature in most engines. So as not to become too technical, the SAE or Society of Automotive Engineers, says a 40 weight oil must have a viscosity at 100 degrees C (212 degrees F) of at least 12.5 centistokes (cSt) but less than 16.3 cSt. Also, the oil’s high temperature/high shear viscosity must also be at least 3.7 centipoises (cP) at 150 degrees C (302 degrees F).

The most important thing to understand is that any 40 weight oil must meet these requirements, whether it’s a 5W40, 15W40, or a straight 40 weight motor oil. In other words, when the motor reaches its operating temperature, its viscosities should all be equal.

Why Run a Straight 40 Weight Motor Oil?

At this point, you may be thinking: why would anyone consider running a straight weight oil since its viscosity is the same as a multi-viscosity oil at engine operating temperatures, yet a straight weight oil will not deliver the same dry/cold temperature advantages? The reason a straight weight oil might be used has to do with shear stability.

To manufacture a multi-viscosity oil, a chemist utilizes chemicals that are known as viscosity index improvers. VI improvers are unique in that they allow the oil to flow differently at different operating temperatures. As an example, a 15W40 diesel engine oil works like a 15 weight oil when cold and a 40 weight oil when hot. As already discussed, this feature of the oil gives it superior flowability at cold dry start-ups which vastly enhances the oil’s ability to provide maximum wear protection.











The unfortunate situation in the engine oil marketplace is that there are vast differences in quality when it comes to multi-viscosity engine oils and the quality of the VI improvers that go in the making of these multi-vis oils. Low quality VI improvers typically found in conventional oil for diesel engines tend to shear or degrade when put under stress. VI improvers, when exposed to the pressure, stress, and heat generated by the cam, pistons, crankshafts, and other engine components that turn at several thousand RPM will tear apart and literally shear these VI improvers.

The more extreme the engine operating environment, the more valuable quality and high-performance additives in engine oil are demanded.

The end result of VI improvers shearing will mean that instead of the oil having a viscosity of 40, it becomes a 35 weight, and then progressively less over time. Straight weight motor oils would be a good choice for extremely high revving race cars such as those that rev over 10,000 RPMs or higher.

For example, top fuel dragsters and funny cars typically run straight weight SAE 60 and SAE 70 due to their engines producing over 3,000 horsepower, along with the type of fuel they run, which contains nitro-methane. In the case of commercial diesel engines, a straight SAE 40 motor oil served its purpose years ago when multi-viscosity technology was new and not quite as developed as it is in the 21st century.

Today, it is possible to find multi-viscosity 15W40 diesel engine oils with shear-stable VI improvers that stay in grade over very long drain intervals. If your interest has been piqued and you’re seriously considering switching from a straight 40 weight motor oil to a 15W40 synthetic diesel oil, carefully research and look only for the highest quality engine oil available and critically make sure the VI improvers that are formulated into the oil are as shear-stable as possible.

When making such a critical decision, the safest choice is always to upgrade to the best.

The Difference Between Conventional and Synthetic Engines Oils

The vast majority of straight 40 weight motor oils are refined from crude oil. These conventional petroleum-based lubricants contain contaminating elements such as nitrogen, sulfur, oxygen, and metal components such as vanadium and nickel.

These unwanted ingredients found in crude oil cannot be completely eliminated through the refining process. When refining crude oil so that petroleum base stocks are created, the process separates the various types of molecules in the oil by weight. This refining process leaves molecules similar in weight, but dissimilar in structure, thus offering reduced performance.

On the flip-side of the coin, you’ll find synthetic engine oils. Synthetic engine oil base stocks are chemically engineered to create pure lubricants. These 100% pure synthetic base stock oils are utilized to create synthetic oil for diesel engines and contain no contaminants or molecules that are not engineered to serve a designed purpose.

Synthetic base stock oils are carefully engineered to have uniform molecular structures that are pure and extremely versatile. They are specifically designed to deliver superior friction reduction, outstanding cold temperature flowability, enhanced fuel mileage, incredible film strength, and an ability to operate over a vast temperature range that conventional lubricants simply cannot match.

A 15W40 synthetic diesel oil will outperform a straight SAE 40 motor oil in engine wear protection, cold temperature flowability, friction reduction, and fuel efficiency. Your diesel engine will last longer and better.

Engine Wear Protection with a 15w40 Synthetic Diesel Oil

There is only a thin microscopic film of oil that keeps the metal surfaces of engine components in your motor from coming into contact with each other and causing unnecessary wear. A 15W40 synthetic diesel oil utilizes resilient 100% synthetic base stock oils that create a far more tenacious and tough fluid film than a conventional straight 40 weight oil can provide. Expect a reduction in wear and far longer diesel engine life from a high-performance synthetic oil.

When temperatures get colder, a straight 40 weight motor oil will thicken up and flow much slower, leaving your critical engine components unprotected for a short period of time during dry cold starts. A 15W40 synthetic diesel oil offers superior cold temperature flowability that far exceeds conventional oils. This superior cold-weather flowability means that critical engine components are lubricated with a protected film immediately, thus providing the necessary engine protection and reduced wear during cold dry starts.

Hot Temperature Performance

Hard-working diesel engines are exposed to extreme operating conditions, and temperatures can reach up to 230 degrees Fahrenheit, and may even reach higher temperatures when exposed to heavier loads. When a diesel engine oil is exposed to this type of heat, the motor oil can quickly break down, thus causing sludge, oxidation, and other deposits.

Independent testing has shown that a high-performance 15W40 synthetic diesel oil can withstand these high temperatures and keep an engine up to 5 times cleaner than a conventional straight 40 engine oil. Such a synthetic diesel oil offers better protection against the formation of performance-robbing deposits, such as oxidation and sludge.

Improved Fuel Efficiency

As mentioned before, conventional petroleum-based diesel engine oils are formulated with base stock oils that contain molecules of different weights and sizes. Also, they contain unwanted elements and wax. These facts mean that a conventional oil requires much more energy to be expended to get the oil to flow and circulate throughout the engine. The extra energy expended means wasted fuel.

On the other hand, a high-performance 100% pure synthetic diesel motor oil is formulated with laboratory engineered base stock oils that contain perfectly uniform molecules, all of the same size and weight. These highly engineered base stock oils naturally offer superior flowability and these molecules flow over one another friction-free and contain no unwanted byproducts. Less energy is required to get the oil circulated throughout the engine. This means a reduction in friction and an improvement in fuel economy.

Reduced Oil Consumption

Conventional or petroleum-based engine oils are less thermally stable and more volatile when exposed to high engine temperatures. As the oil tends to burn off, the potential for too much motor oil being consumed is a real possibility. This could lead to there being insufficient oil in the engine, which could lead to certain complex parts of the engine having less oil available for lubrication and protection.

Again, a 100% pure synthetic diesel engine oil offers far superior high-temperature thermal stability and reduced oil consumption.

Reduced Emissions and Environmental Conservation

Synthetic engine oils are specifically engineered to last longer, improve fuel economy, and reduce environmentally damaging emissions. In other words, synthetic motor oils are environmentally friendly. They burn off far less than conventional oils, they last up to 8 times longer than conventional oils, vehicles that run synthetics burn less fuel, tailpipe emissions are much less, and pure synthetic oils generate less waste oil.

If you’re interested in environmental conservation, then switching to a 15W40 synthetic diesel oil is an excellent choice.


Hopefully, this blog post has provided enough information in helping you make a decision on whether to stay with a straight 40 weight motor oil or to switch to a 15W40 synthetic diesel oil.


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