The difference between 10w30 and 10w40 is the thickness of the oil at operating/high temperatures, at which 10w40 is thicker than 10w30.
In This Guide
The first viscosity number, “10” in the case of both of these oils, rates how easy it pours at starting/low temperatures, such as starting the engine during winter. The lower this number is, the easier it pours when cold, so an oil rated “5” will pour faster than one rated “10.”
The second number, “30” and “40” for these oils, is a rating of how easily the oil flows at operating temperatures and in hot weather. The higher this number is, the better it adheres to and protecting components under extreme pressure and heat.
When to Use 10w30 vs 10w40
When choosing an oil, you should always use the rating specified by the engine manufacturer to ensure adequate lubrication of all internal components. Most manufacturers specify a range of viscosities that can be used, letting you choose one that best fits your driving conditions.
Using 10w30 oil in cold weather will help reduce excessive oil temperatures and drag as the engine warms up. Using 10w40 oil in the summer will help the oil stick to internal components in high temperatures, avoiding the wear and tear from metal-to-metal contact between moving parts.
As previously mentioned, the only difference between 10w30 and 10w40 oil is their thickness at engine operating (hot) temperatures. They will flow at the same rate during a cold startup since the starting viscosity is “10” for both formulations.
High Mileage Vehicles
In the past, it was common practice to switch to a higher viscosity oil as an engine got older, for example, using a 20w50 oil in place of the 10w40 recommended by the manufacturer. As oil passages wore down from friction and expand, switching to a thicker oil helped the oil pump push oil through them, ensuring complete lubrication.
However, in modern engines, this is not a good idea. Improvements in machining, oil chemistry and oil filtration means these oil passages remain the same size throughout the life of the engine. Most burning of oil comes not from increased gaps between components, but from aging seals. Using a higher viscosity oil increases strain on the oil pump, and it may even prevent the oil from circulating adequately through the entire engine. Oil manufacturers have developed high mileage oils to help deal with seal problems in modern engines while still providing the same lubrication performance the motor was designed for.
How Oil is Graded
Most motor oil produced today is multigrade oil. Normally, oil will thin out as temperature increases, but multigrade formulations have additives that cause it to thicken as it heats up to better fit the range of operating conditions seen in car engines. The viscosity numbers used to grade oils are based on a test developed by the Society of Automotive Engineers (SAE) in which the oil’s flow rate is timed as it passes through a length of tubing.
The flow rate of multigrade oils is first tested at 0 degrees Fahrenheit. The number of seconds it takes to flow through the tubing is the number used before the “w.” That letter stands for “winter,” since it’s a rating of cold weather performance.
The oil is tested again after it has been heated to 212 degrees Fahrenheit. The time it takes to flow through the tubing is used for the number after the “w.” A 10w40 oil takes 10 seconds longer to flow through the tube during this test than a 10w30 oil.
Single Grade Oil
In the past, oil manufacturers could only produce oils that would flow at a single rate, requiring oil changes to match the oil viscosity to the current weather conditions. Heavier oils that offered good protection in the summer would be too thick to pump in the winter, and light winter formulations wouldn’t protect parts during the summer. The viscosity of these oils is stated as “SAE” followed by the weight. These oils have fallen out of use in engines, but they’re still commonly used as lubricants in differentials and electric motors.
What to Read Next
- 8 Best Engine Oils
- Synthetic vs Conventional Engine Oil
- How Much Does an Oil Change Cost
- How Long Does an Oil Change Take
- Best Place for an Oil Change Without Being Ripped Off
Over to You
We’re interested to know – which of these oil ratings did you use in your vehicle and what was the temperature outside at the time? Let other readers know by leaving a comment below!