- Engine oil viscosity grade indicates the viscosity of engine oil. It’s usually made up of two numbers and a W. The first number indicates the oil’s cold weather viscosity grade, the ‘W’ stands for winter, and the last number indicates the oil’s hot weather viscosity grade.
- 5W-40 is the suggested oil for most diesel applications, while 5W-30 oil is the recommended oil for many modern gasoline applications.
- You can find out which oil your vehicle uses by checking your owner’s manual or the oil fill cap on top of the engine.
When you go shopping for engine oil, you’ll likely run across a variety of grades, including 5W-30 and 5W-40. What’s the difference between these two products—and which should you put in your car?
Let’s find out.
What’s an Engine Oil Viscosity Grade?
Before we go any further, it’s important to discuss engine oil viscosity grades. The grades are defined by the Society of Automotive Engineers (SAE) and listed on the oil container. Each grade indicates a specific viscosity characteristic and range.
Viscosity refers to an oil’s thickness and, therefore, its resistance to flow. Engine oil that is thinner flows more easily than thicker oil. Also, the thickness of the oil changes with temperature (oil is thicker when it’s cold).
When you look at a bottle of multi-grade oil, you’ll notice the viscosity grade contains two numbers and a ‘W’. For example, the oil might have a viscosity grade of 5W-30.
Here’s what the viscosity grade means:
- The first number is the oil’s cold weather viscosity grade
- The ‘W’ stands for winter
- The third number is the oil’s hot weather viscosity grade
The larger the viscosity numbers are, the thicker the oil and the greater its resistance to flow. For instance, 5W-30 oil is thicker than 5W-20.
It’s worth noting that there are also single-grade oils that only have one number. An example would be SAE 20 oil or 30W oil. Such oils have a narrow viscosity range and are not suited for modern automotive applications.
5W-30 Oil vs. 5W-40 Oil
Now that you understand oil viscosity, you probably have an idea of the difference between 5W-40 oil and 5W-30 engine oil. Although both have the same viscosity grade in cold weather, 5W-40 is thicker than 5W-30 at warmer temperatures.
It’s also worth pointing out that:
- 5W-40 is the suggested oil for most diesel applications. Rarely (if ever) do vehicle manufacturers recommend putting 5W-40 oil in a gas-powered engine.
- 5W-30 oil is the recommended oil for many modern gasoline applications.
Which Grade of Oil is Correct for Your Car?
There are two ways to determine the correct engine oil for your car. The first option is to look the information up in your owner’s manual. Another way is to look at the oil fill cap on top of the engine.
You should use the type of oil the vehicle manufacturer recommends for your car, regardless of whether that’s 5W-30, 5W-40—or something else altogether. Using the incorrect type of oil can cause engine performance problems and trigger the check engine light. The practice can even accelerate engine wear and eventually cause internal damage.
The only time you might consider deviating from the oil the vehicle manufacturer recommends is when your car’s engine already has internal problems. If the engine is suffering from increased internal clearances due to wear, choosing a higher viscosity may compensate for those clearances enough to limp the engine along for a short while. But you should only attempt this method on an engine that’s already headed for the scrapyard.
In every other instance, you should use the type of oil the vehicle manufacturer recommends. Doing so will keep your engine running better, longer.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.