Dexos2 vs. Dexos D: What Changed and What Comes Next for Diesel Engine Oil
Engine oil specifications tend to receive attention only when an owner's manual changes viscosity recommendations or a familiar logo disappears from the bottle. Behind those changes, however, is a continuing engineering effort to balance engine durability, fuel economy, emissions-system protection and increasingly demanding operating conditions.
General Motors' transition from dexos2 to dexos D is a good example.
Dexos2 was already a sophisticated specification designed for modern diesel engines equipped with particulate filters. Dexos D did not replace it simply because dexos2 was inadequate. Instead, GM developed a dedicated light-duty diesel specification capable of supporting a newer generation of engines, including applications using considerably lower-viscosity oil.
Now the diesel-oil industry is preparing for another major change. New API heavy-duty diesel categories arrive in 2027 with greater emphasis on thermal stability, lower ash and phosphorus levels, aftertreatment durability and, in some applications, even lower viscosity.
Taken together, these developments provide a useful look at where diesel engine oil is heading.
What Was dexos2?
GM introduced dexos2 as a global service-fill engine oil specification for passenger cars and light-duty trucks with diesel engines, while also allowing its use in certain gasoline applications depending on the market.
The underlying GM specification, GMW16177, includes SAE 0W-30, 5W-30, 0W-40 and 5W-40 viscosity grades. GM's current documentation continues to identify oils meeting the specification as dexos2.
One of the defining characteristics of dexos2 was its compatibility with modern exhaust aftertreatment systems.
Diesel particulate filters introduced a problem that older engine oils did not have to address to the same degree. A small amount of engine oil is inevitably consumed during normal operation. When that oil reaches the combustion chamber, some components of the lubricant can eventually make their way into the exhaust.
For that reason, dexos2 belongs to what is commonly described as the lower-SAPS family of lubricants. SAPS refers to sulfated ash, phosphorus and sulfur.
Typical dexos2 requirements include sulfated ash of no more than 0.8 percent and phosphorus between 700 and 900 ppm. Traditional dexos2 formulations also generally have an HTHS viscosity of at least 3.5 cP. HTHS, or high-temperature high-shear viscosity, is an important measurement of how an oil behaves in highly loaded areas of an operating engine.
This produced a familiar type of diesel lubricant: often a 5W-30 or 5W-40, relatively high in HTHS viscosity, but with controlled ash, phosphorus and sulfur content to accommodate emissions equipment.
Dexos2 therefore should not be thought of as an outdated specification from the era before modern emissions systems. It was specifically designed to work with technologies such as diesel particulate filters.
Why GM Created dexos D
GM eventually developed a separate specification specifically for its light-duty diesel engines: dexos D, identified internally as GMW18006.
The distinction is important.
Dexos D is not simply another viscosity grade. The dexos designation defines a set of performance requirements, while designations such as 0W-20 and 5W-30 describe viscosity.
That distinction allowed GM to develop an oil specification around newer diesel engine requirements while supporting significantly lower viscosity grades.
This became particularly visible with GM's 3.0-liter Duramax diesel applications, where 0W-20 dexos D became a familiar recommendation.
Lower-viscosity oil can reduce internal friction and pumping losses, contributing to improved efficiency. Achieving that while maintaining adequate protection in a turbocharged diesel engine requires the engine and lubricant to be developed around the same objectives.
At the same time, dexos D still has to address the requirements expected of a modern diesel lubricant: wear protection, oxidation resistance, deposit control and compatibility with emissions aftertreatment.
That is an important way to understand the change from dexos2 to dexos D.
It wasn't simply:
Old oil → better oil.
It was a change in the performance envelope GM wanted from its diesel lubricants.
dexos2 and dexos D Are Not Just Different Viscosities
It is easy to associate dexos2 with 5W-30 and dexos D with 0W-20, but that is no longer accurate.
GM's current dexos D licensed-oil database contains both 0W-20 and 5W-30 products. GM currently lists oils such as Castrol EDGE 5W-30 C3, Havoline ProDS D 5W-30 and Petronas Syntium 5000 XS 5W-30 as dexos D licensed products alongside numerous 0W-20 formulations.
GM went another step in August 2024 when it announced that 0W-40 and 5W-40 would also be added to the dexos D specification.
Interestingly, GM required dexos D formulations in those two viscosity grades to display both the dexos D and dexos2 logos.
That tells us something important about the specification.
dexos D does not mean 0W-20.
It is a performance specification that can encompass substantially different viscosity grades.
The correct viscosity still depends on the particular engine and application. The presence of a dexos D approval on a 5W-30 or 5W-40 does not automatically make that viscosity appropriate for an engine whose owner's manual specifies 0W-20.
What Happened to dexos2?
GM announced in June 2023 that it intended to sunset dexos2 and stated specifically that dexos D would replace dexos2 for GM diesel engine applications.
GM subsequently modified that plan.
In October 2023, the company rescinded its previously announced stop-production and stop-sale dates.
Existing approved dexos2 formulations were allowed to continue being produced and sold, but GM stated that it would accept no new dexos2 submissions.
New formulations would instead need to be submitted under specifications including dexos D.
That is why dexos2 oil has not simply disappeared from store shelves.
Nevertheless, GM's current consumer guidance is clear: dexos2 has been replaced by dexos D for light-duty diesel engines.
GM also states that dexos D oils meet or exceed dexos2 performance and are backward compatible, provided the appropriate viscosity grade is used.
In other words, dexos2 remains relevant, but GM's diesel oil development has moved to dexos D.
The Next Major Diesel-Oil Change Arrives in 2027
Another useful indication of where diesel oil technology is heading comes from the heavy-duty side of the industry.
This is separate from dexos D, which is a GM light-duty diesel specification, but many of the engineering challenges are related.
The industry initiative known as PC-12 will result in two new API heavy-duty diesel oil categories beginning around January 1, 2027: API CL-4 and API FB-4.
CL-4 is intended to replace CK-4 and maintain broad backward compatibility.
FB-4 succeeds FA-4 and is intended for newer engines capable of using lower-viscosity lubricants.
Chevron says FB-4 opens the door to XW-20 heavy-duty diesel oils for engines designed to use them.
Oil viscosities that would once have seemed unusual in a heavy-duty diesel are becoming possible as engine architecture and lubricant technology develop together.
But viscosity is only part of the change.
The Next Challenge Is Protecting the Emissions System
One of the most significant directions in the next generation of diesel lubricants is reducing the effect engine oil has on exhaust aftertreatment.
A diesel particulate filter collects two very different materials: soot and ash.
Soot can largely be oxidized during regeneration. Ash cannot.
Some of that ash originates from lubricant additives after small amounts of engine oil are consumed. Over time, the noncombustible material accumulates in the DPF.
That creates a difficult problem for lubricant engineers because many traditional additives perform valuable jobs inside the engine.
The goal is therefore not simply to remove additives. It is to maintain wear protection, cleanliness and oil life while reducing the amount of material that can negatively affect the emissions system.
Chevron describes the forthcoming CL-4 and FB-4 requirements as having a tighter chemical box with reductions in sulfated ash and phosphorus intended to help aftertreatment systems operate properly over a longer service life.
This represents an important change in how engine oil needs to be viewed.
The lubricant is no longer responsible only for protecting the engine.
It increasingly has to help protect the engine and the emissions system as a complete package.
Chevron has described this explicitly, arguing that lubricant formulators can no longer treat the exhaust aftertreatment system as something operating in the background.
Heat Is Becoming Another Major Challenge
Modern diesel oils are also being asked to survive increasingly severe thermal environments.
Higher operating temperatures, sophisticated turbocharging, piston cooling and tighter engine designs can expose oil to substantial heat.
Chevron says the upcoming heavy-duty oil specifications include improved oxidation and wear protection to address higher operating temperatures and advanced engine designs.
Oxidation resistance is particularly important because excessive heat can accelerate lubricant degradation, increase viscosity and contribute to deposits.
This means future diesel oil development will likely involve several objectives that sometimes compete with one another:
Lower friction for efficiency.
Strong oil films where the engine requires them.
Reduced ash and phosphorus for emissions-system durability.
Greater resistance to oxidation and extreme temperatures.
Improved deposit control.
Adequate protection over increasingly demanding operating conditions.
Achieving all of those simultaneously is considerably more complicated than simply selecting a thicker or thinner oil.
What Could Come After Today's dexos D?
GM has not announced a next-generation dexos D specification, so it would be premature to claim that a "dexos D Gen 2" is coming.
There are, however, some clear industry trends.
First, there is little evidence that the future can be summarized as simply thinner oil.
GM itself has expanded dexos D beyond 0W-20 to include 5W-30 and, more recently, 0W-40 and 5W-40.
At the same time, the heavy-duty diesel industry is developing XW-20 oils for engines specifically designed to take advantage of their lower friction.
Those developments may appear contradictory, but they really demonstrate the same principle:
The engine determines the viscosity requirement. The oil specification determines the performance the lubricant must deliver.
Second, aftertreatment protection appears likely to become increasingly important.
The upcoming heavy-duty specifications place additional emphasis on lower sulfated ash and phosphorus because emissions systems are expected to remain effective for longer periods.
Similar engineering pressures exist in light-duty diesels equipped with DPF, SCR and increasingly sophisticated emissions monitoring.
Third, thermal stability is likely to remain a major development area.
New engines are being asked to produce more power, improve efficiency and meet increasingly demanding emissions requirements simultaneously.
Lubricants must survive those conditions without excessive oxidation, deposits or loss of protection.
Diesel Oil Is Becoming More Specialized, Not Simply Thinner
The transition from dexos2 to dexos D illustrates how much diesel engine oil has changed.
Dexos2 provided GM with a robust, lower-SAPS lubricant specification suitable for diesel particulate filter-equipped engines and was commonly associated with higher-HTHS 5W-30 and 5W-40 oils.
Dexos D created a dedicated path for GM light-duty diesel engines and enabled applications using considerably lower viscosity while still requiring the performance necessary for modern diesel hardware and emissions systems.
Now the broader diesel industry is preparing for another step.
The next generation of oils will be expected to tolerate greater thermal stress, maintain engine durability, reduce the amount of material that can affect exhaust aftertreatment and, where the engine is designed for it, reduce viscosity and friction even further.
At the same time, GM's expansion of dexos D into 30- and 40-grade oils shows why it would be a mistake to assume every future diesel will simply use thinner oil.
The larger trend is more interesting.
Engine oil is becoming increasingly application-specific, while its responsibility is expanding beyond protecting the engine itself. Future diesel oils will increasingly be expected to protect the engine, turbocharger and emissions system as one integrated system.
That may ultimately prove to be the most important difference between the diesel oils of the past and those that come next.
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