The type of stresses that diesel and gasoline engines impose on the engine oils are fundamentally different, and thus diesel and gasoline engine oils have been developed to fit the unique requirements of performance. Most users assume that engine oil is engine and diesel and gasoline engine oils can be used interchangeably, but this utilization may undermine the engine protection and emission. Replacement of incorrect engine oil formulation may hasten the wear, deposit growth and reduce the engine life.
In this guide, we shall discuss the mechanical and combustion variations that are the sources of these formulation requirements, deconstruct some of the important technical differences, and draw practical conclusions on the selection. To have the full selection of engine oil grades that can be applied to different applications, see our full range of engine oil grades.
Fundamental Differences Between Diesel and Gasoline Engines
The cause of the oil differences can be seen in the way these engines produce power and in the conditions of operating.
Diesel engines use compression ignition, in which air is compressed in very high ratios, which is usually 14:1 and 22:1 which produces an adequate amount of heat to spontaneously combust fuel. This gives greater maximum pressures and temperatures and a great deal of soot due to uncombustion within rich areas. The gasoline engines are spark ignited, have lower compression ratios (typically 8:1 to 12:1), are operated at higher RPM and have a cleaner combustion, however they have drawbacks such as degeneration of fuel through frequent cold starts and low-speed pre-ignition (LSPI) in recent direct-injection models.
The existence of these distinct environments poses distinct lubrication problems.
| Engine Type | Operating Characteristics | Primary Lubrication Challenge |
| Gasoline | High RPM, lower compression, spark ignition, minimal soot | Oxidation stability, LSPI protection, fuel economy |
| Diesel | High compression, high load/torque, compression ignition, high soot | Soot control, deposit management, extreme wear protection |
These fundamental differences imply that diesel oils are to be provided with greater resistance to severe contamination whereas gasoline oils are to become more efficient and to be equipped with the special precautions of the modern engines.
Why Diesel Engine Oil Requirements Are More Demanding
The combustion process of diesel engines produces much more soot and other harmful acids byproducts, and thus it puts much pressure on the capacity of the oil to keep clean and neutralise the acids.
To maintain the soot suspended and avoid the rise in viscosity, sludge, or abrasiveness, diesel formulations necessitate a high concentration of more soot-controlling additives such as higher proportions of dispersant additives, 2-3 times that of gasoline oils. They also have a larger TBN (usually 10-15 mg KOH/g or higher) to counteract sulfuric and other acids generated in combustion, to allow longer drain intervals where heavy usage is required.
To get a closer examination of the latest diesel standards such as API CK-4 which focuses on enhanced oxidation stability, shear resistance as well as soot control, refer to our guide on diesel engine oil requirements.
Gasoline Engine Oil Requirements Explained
Gasoline engines that run at high speed, produce less soot, have greater focus on the fuel economy and emissions require oils that have been optimized to address other priorities.
The current formulations of gasoline focus on low-speed pre-ignition (LSPI) defense in turbocharged, direct-injection engines, shorter timing chain wear, and improved fuel economy by way of decreased viscosity and friction modifiers. They also concentrate on deposits and varnish control in high-RPM conditions and additive packages with balanced additive packages that would not interact with catalyst converters.
Learn more about geographical and market-based concerns in the choice of the correct formulation in our article on oil requirements in gasoline engine oil requirements.
Additive System and SAPS Level Differences
Among the most essential differences one is the additive chemistry, and more specifically on the additive levels of Sulfated Ash, Phosphorus and Sulfur (QS10) as well.
Diesel oils frequently permit larger proportions of SAPS, to sustain robust detergent and anti-wear schemes required to control soot and acid but some current lower-SAPS (such as achea C categories) types trade off this with aftertreatment compatibility. In passenger vehicles, gasoline oils have lower SAPS to cover delicate three-way catalysts and particulate filters and sacrifice some detergency to emission system life.
This is another trade-off that makes formulations non-interchangeable: high levels of SAPS caused by diesel oil might poison gasoline catalysts, and low levels of SAPS due to high amounts of gasoline oil can result in inadequate protection in an environment that builds up soot early. On low-SAPS formulations and how they are used in current engines, see our post on diesel vs gasoline engine oil differences.
Viscosity and Duty Cycle Considerations
Selection of viscosity is directly proportional to the duty cycle of the engine, the operating temperatures, and its load requirements.
Diesel engines which usually operate under constant heavy loads prefer the oils which keep the films strong even at high temperatures, at times the engines require higher viscosity such as 15W-40 grade to be used over heavy loads. Gasoline engines have more frequent cold starts and are focused on the efficiency, therefore usually lower viscosity (e.g., 0W-20 or 5W-30) to achieve better pumpability and lower friction.
Climate and work also play a role: good low-temperature flow is needed in colder regions whereas thermal stability is needed in hot, high-work areas. The viscosity always must be matched to the duty cycle of the particular mode of operation. This detailed article provides practical advice on comparing the viscosity and climate friendliness: choosing the right oil based on engine type and duty cycle.
Common Mistakes When Using One Oil for Both Engine Types
The most common mistake is to believe that a single can of oil can be used on both engines particularly when in times of shortages or price control in the market.
Many people may consider short-term substitution to be an innocent action but over time when people use gasoline oil in the place of diesel the accumulation of soot may also become excessive, so may be the viscosities, wear, and the filter clogging. In a gasoline engine, on the other hand, diesel oil will be prone to LSPI, catalyst ruin owing to elevated levels of SAPS/detergents, or deposition challenge due to the incompatibility of additives.
These errors are usually based on over-generalization and therefore may lead to expensive reparation costs. They should be shunned by distributors and technicians through product segmentation. To understand the common pitfalls in sourcing, read our article on engine oil for diesel and gasoline engines.
Portfolio Planning for Distributors and OEM Buyers
It is necessary to have distinct products of diesel and gasoline oils that cater to different market needs without confusion.
The distributors will enjoy the differentiated products which meet the needs of different customer segments heavy-duty fleets need strong diesel formulations, and passenger car importers are in need of LSPI-protected gasoline oils. Different products may be used wrongly and lost trust.
Proper portfolio will help in making clear recommendations as well as adhering to the regional standards. Find out further on balancing in this guide on balancing between OEM and branded approaches to engine oil portfolio.
Special Cases — High-Mileage and Mixed-Use Vehicles
Older engines or vehicles that have mixed gasoline/diesel duty cycles (particularly some light duty pickups) are also a special problem.
High-mileage engines are perhaps the reason why the formulations of enhanced seal conditioner and deposit control are applied, but the fundamental requirement is based on the initial engine type. Where mixed-use occurs, there are some mixed-purpose oils on balance, but specialized oil is still the best choice to make.
Use caution in terms of consultant guidelines when formulation balance occurs. To further protect old engines, check out our discussion on explore our engine oil solutions for different vehicles.
Conclusion — Engine Design Determines Oil Design
Oil design is eventually determined by the engine design. These unique stresses, whether it is soot and acids, LSPI and efficiency requirements, make diesel and gasoline engines conditional to differing oils, with different additive systems, with different levels of SAPS, performance values. Always choose on the basis of the specifications and the actual conditions of the engine so as to be reliable and durable.
Correct selection would not only avoid premature wear but also assist in meeting the requirement of emissions and length of service. To individuals who may be looking at the custom formulations or sourcing strategies our information on engine oil solutions offer further practical guidance.