Differences between 15w50 vs 10w40 engine oil

Engine oil plays a vital role in a car’s engine. Without engine oil, it will not be easy for engines to run. However, not all engine oils are suitable for your vehicle as they have different qualities and capabilities. Engine oils are ground-level informants in the automotive industry, although they get no credit for their contributions to performances. 

But besides that, without them, there wouldn’t be internal combustion. Modern motor engines can compress air and burn small amounts of fuel to get power. What enables this is well-prepared engine oil which oil engineers have taken time to study the properties of various types of synthetic oils’ natural and naturally derived contents. 

There are various engine oils in the market. In this article, I will compare 10w40 and 15w50 motor oils to see their differences and similarities. The oils have different properties and working capabilities. 

10w40 engine oil properties

10w 40 refers to the weight or viscosity of engine oil as per the Society of Automotive engineers. The oil features a viscosity grade of 10 W at both low temperatures and above forty. This means that this oil becomes thick in cold conditions but thin when heated up. This oil type does not gain viscosity after heating up. In other words, the oil behaves precisely as a 10W weight oil while cool and like forty weight oil while hot. 

The 10W rating 

10W represents the oil’s viscosity while cold. All lubricants have specified viscosity that they can maximally reach cooler temperatures. The higher the W number, the thicker the oil whereby W is for Winter. Therefore, in the case of 10 W, the oil will be more viscous during Winter than an oil rated at 5W engine oil. 

The 40 rating

This is the oil’s viscosity at hotter temperatures. This explains how best oil can flow in an engine at 212 Fahrenheit (100 degrees Celsius). The hot viscosity rating focuses on the lubricant’s capacity to protect the engine’s components and seal leakage while in a thinner state. 40 is the high-temperature viscosity during hot conditions (summer). 

Temperature range

The recommended ambient temperature range for 10W – 40 engine oil is between -30 °C (-22 °F) at low temperatures and +40 °C (104 °F) at high temperatures. 

The 10W40 engine oil API

The 10 W 40 engine oil’s API is CI-4/SL.

Uses of 10W -40 engine oil 

Although this oil is not recommended for modern-day passenger vehicles, it is still popular with heavy and heavy-duty gasoline engines on light trucks. The oil’s weight is primarily used in smaller motorcycle motors or diesel engines. 

The oil’s viscosity functions as a substitute for older motors with oil leaking or burning issues. This is because it has a thicker viscosity when the engine is hot. Therefore, this enables it to lubricate old engine moving components at high mileage without easily leaking. 

Moreover, the oil’s thicker viscosity makes it a good option for motors whose oil temperatures are high as it will resist the high temperatures thermal breakdown.

10W-40 is good for smoother startup protection because it is synthetic, and synthetic oils flow better than conventional engine oils and maintain sufficient viscosity for piston bearing and skirts’ protection at high temperatures. 

15W50 engine oil 

15W50 motor oil comprises semi-synthetic oils alongside a few selected additives. This oil is developed for use in modern four-stroke engines, guaranteeing optimum performance for all motors.

The 15W50 is the SAE class to which the motor oil is classified. The oil’s SAE class describes its viscosity, like the flow properties based on the working temperatures. 

The motor oils of class 15W50 are multi-grade oils that possess excellent flow properties at both warm and cold temperatures. 15W is the oil’s ability to flow at cold temperatures, whereas W is for Winter. 

Even at low temperatures of -25 degrees celsius (-13 °F), this oil can still be pumpable to produce enough lubricant to flow in the engine. The 50 is the engine oil’s ability to cruise at an operating temperature of one hundred degrees celsius. 

15W50 oil viscosity index

At one hundred degrees Celsius, the oil’s viscosity ranges from 16.3 to 21.8mm2/s. 

15W50 Temperature range

15W50 engine oil grants complete protection on a wide range of temperatures. The oil can flow even at temperatures of -13 degrees Fahrenheit or -25 degrees celsius and can still withstand temperatures as high as 131 degrees Fahrenheit.

Therefore its temperature range is -13°F to 131°F. This helps the oil prevent unnecessary deposits from building up in the engine, giving an engine longer life. 

Differences between 10w40 vs. 15w50

While the two oils can be excellent in usage, they have some differences. They include:

  • The 10W 40 oil has a better flowability at cold temperatures than the 15W engine oil. This is because the 15W -50 oil is thicker at low temperatures than the 10W-40 so, it will not flow well at startups. However, it is best at high temperatures because it is thicker and therefore maintains its viscosity even when temperatures rise. 
  • Because it is thinner, the 10W40 flows well at high temperatures leaving no residue in the engine. 
  • The 15W-50 engine oil is better for use during Winter than the 10W-40. 
  • 10W -40 is denser or thicker than what most engine manufacturers recommend. 15W, on the other hand, increases wear fuel consumption and reduces power. So, it is better to consult the car manual as it will indicate which oil is better for your engine. 

What happens if you put oil that the car manufacturer does not recommend?

Using a different oil from the one recommended for your car engine can lead to transmission failure, poor lubrication, and overheating. Therefore it is essential to use the exact oil that your car’s manufacturer recommends to avoid damages and losses since even a mechanic cannot repair a damaged engine due to the wrong oil. 


Engine oil is vital in ensuring the smooth running of the engine. However, not all engines are ideal for all car engines. When comparing the 10W40 and the 15W50 engine oil, they are both excellent use on engines they are recommended for.

15W50 engine oil is best for cars that operate at high temperatures, like racing cars. Because of its thickness at colder temperatures, it might not be ideal for use during Winter.

Scroll to Top