Oil plays a critical role in lubricating the moving parts inside of gasoline and diesel engines. Often, the words motor oil and engine oil are used interchangeably. Though, for the most part, they are exactly the same thing.
The one possible distinction is that diesel engines often have specially formulated engine oils with extensive additives. These oils are optimized with viscosity and other additives that are ideal for how a diesel engine operates.
Yet even though motor oil and engine oil are essentially the same and serve the same purpose, there are tons of differences in viscosity and special additives that can determine which one is best for your car. To help demystify the difference between motor oil, engine oil, and synthetic oil, we’ll need to examine what makes them different closely.
Additives in Motor Oil & Engine Oil
Engine oil or motor oil is essentially refined from crude oil. The refining process optimizes the viscosity for lubricating the moving parts inside an internal combustion engine. Roughly 95% of the oil is the base oil, and another 5% is special additives.
Common Motor Oil Additives
Most modern motor oils have special additives included in them to help with performance and/or to meet specific needs for different types of engines. Generally, these additives serve three purposes. Some enhance the base oil’s existing properties by adding antioxidants, corrosion inhibitors, or anti-foam agents. Some also add demulsifying agents.
Some motor oil additives are specifically formulated to suppress undesirable base oil properties. This includes pour-point depressants and additives that improve the viscosity index (VI).
The third type of motor oil additives is designed to imbue the base oils with new properties. This often includes extreme pressure (EP) additives, detergents, metal deactivators, and agents that affect the oil’s tackiness.
What Makes Diesel Engine Oil Different?
Diesel engine oil has a higher percentage of additives by volume. This includes a higher amount of base oil detergent additives specially formulated to neutralize acids and clean engine components.
Diesel engine oil is typically formulated with higher anti-wear additives in the form of zinc dialkyl dithiophosphate. This is because the catalytic converters in diesel systems are designed to be able to deal with combustion by-products in a way that gasoline engines don’t.
Diesel engines tend to produce more soot and other combustion byproducts that tend to find their way into the crankcase, where they can interact with the oil. Without these additives, the oil would break down and not have the same lubricating properties a diesel engine needs for long-term performance.
If you were to put these same additives in a typical gasoline engine, it would have a devastating effect on performance. In a gasoline engine, these detergent additives would start to clean the cylinder walls, harming the seal between the rings and liner. In no time, the gasoline engine could suffer from compression loss and efficiency. It could also start to clog your catalytic converter and other parts of your exhaust system.
The Importance of Viscosity
Viscosity is the most important factor in motor oil for gasoline engines or diesel engines, as it measures the oil’s lubricating properties. The viscosity of any oil for internal combustion engines needs to be pumpable at the lowest start-up temperature. Yet, it also still needs to have the same lubricating properties to protect the engine’s moving parts at in-service temperatures.
For the most part, diesel engine oil has a higher viscosity than motor oils meant for gasoline engines. If you were to put higher-viscosity diesel engine oil in a gasoline engine, you would get more heat from internal fluid friction. This can cause the oil to break down rapidly.
At the same time, high-viscosity diesel engine oils also tend to have more low-temperature pumpability. This means that the oil can be very thick during very cold starts, yet the oil pump can still deliver it to the critical engine components in the lifter valley.
The Difference Between Synthetic & Conventional Oils
Conventional motor oils are made directly from refined crude oil petroleum. Whereas synthetic motor oil is created from artificial chemical compounds that are produced by breaking down and then rebuilding petroleum molecules.
So-called full synthetic oils have a completely synthetic base stock that is specifically formulated at the molecular level. This also means that synthetic motor oils don’t have the same additives that conventional motor oils rely on to prevent degradation. However, these oils are significantly more expensive per volume than conventional oil.
Synthetic blends use a mixture of conventional motor oil as well as synthetic base motor oil. This can provide superior protection than if you used conventional oil by itself.
The Benefits of Synthetic Motor Oil
Most of the synthetic motor oils you find in the automotive industry are polyalphaolefins (PAO). This provides the oil with certain benefits.
Resists Oxidation & Degradation
This starts with superior oxidation resistance and is less likely to degrade due to heat. Synthetic motor oil also better deals with combustion byproducts, fuel contamination, water contamination, metal particles, acids, pro-oxidants, and extreme heat.
Synthetic motor oils also tend to have a naturally higher viscosity index while being more stable despite temperature changes from cold engine startup and operating conditions. Synthetic motor oil is more pumpable and better circulates effectively within the engine at low temperatures.
Synthetic motor oil also tends to have a longer lifespan than conventional motor oil. Most are formulated to last between 5,000 and 7,000 miles. Whereas pure conventional motor oil typically only has a lifespan of around 2,000 to 3,000 miles.
The Drawbacks of Synthetic Motor Oil
For all its strengths, synthetic motor oil has a few potential disadvantages worth considering.
For starters, the process of creating it is more complex and labor-intensive, which naturally increases the cost. On average, you can expect synthetic motor oil to be roughly two to four times that of conventional oil.
Synthetic motor oil also suffers from greater additive precipitation during cold storage conditions. If stored for too long, it can cause special additives to separate from the oil completely.
Engines with multi-grade blended synthetic motor oils also tend to have slightly lower fuel economy at highway speeds.
What Type of Oil Is Best for My Car?
Most automakers provide specific recommendations for the type of motor oil you should use in your car. You can usually find this information in the owner’s manual or the repair guide for your make and model.
The most popular types of motor oil for gasoline engines are 5W-20 or 5W-30 oil for dealing with colder temperatures. However, 10W-30 oil is more common in engines with a higher ambient running temperature.
Modern diesel engines tend to see the most benefits from 15W-40, 5W-40, and 0W-40 synthetic engine oils that are specifically formulated for diesel engines. Not only does it have the superior pumpability that diesel engines need in cold temperatures, but synthetic engine oils tend to have a longer average lifespan.
Frequently Asked Questions
Can synthetic motor oil be made from natural gas?
One of the more recent innovations in the world of motor oil is the ability to create synthetic oil from hydrocarbon chains harvested from natural gas. It is a much cleaner oil, free of the contaminants found in conventional motor oil. The process of refining it is in its early stages, which typically means the price per volume is very high.
Does Engine Oil Affect the Exhaust System?
The type of motor oil additives you use has a more significant impact on the exhaust system of gasoline engines. Internal combustion creates byproducts like lead, zinc, and phosphorus, which can impede the catalytic converter’s ability to control emissions. The high anti-wear additives in diesel engine oils do a better job dealing with combustion byproducts than gasoline engines.
Can I Use Diesel Engine Oil in My Gasoline Car?
You could use diesel engine oil in your gasoline engine only so long as it meets the viscosity specifications for your car. Though this is somewhat rare, diesel engine oil additives aren’t ideally suited for gasoline combustion engines. You also run the risk of clogging your catalytic converter over time.
Can I Use Car Motor Oil in My Diesel Engine?
Motor oil must meet the viscosity and anti-wear specifications to work in a diesel engine. Pumpability can also be a factor if you’re going to run your diesel engine in cold conditions. So, it’s almost always better to use oil specifically formulated to run in diesel engines.
While they are both refined to do the same job of lubricating an engine, there are some notable differences between diesel engine oil and the motor oil used in gasoline engines.
Diesel engine oil tends to be more pumpable, which helps for cold weather start-up. They also tend to have high viscosity and anti-wear additives that optimize diesel performance.
Modern synthetic oils and synthetic blends tend to be better for diesel engines. Though not if you’re going to put the car in long-term cold storage, as the additives in the synthetic oil can precipitate out of the solution.
Motor oils used in gasoline engines tend to be formulated to handle high heat conditions. They also tend to have fewer additives, which doesn’t make them a good fit for most diesel engines.
If you’re unsure about the type of oil to put in your car, you can always check the owner’s manual. They will have specific recommendations on the best oils for your type of engine.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.