10W-30 and 10W-40 are some of the two different types of multi-grade engine oil that are commonly used to lubricate automobile engines. Depending on how you use your vehicle or the outdoor temperature, having the proper motor oil provides numerous benefits, apart from reduced friction among the moving parts inside your engine.

Some people are confused one for the other, so they end up using a different oil in their engines. Others, however, do not know what those alphanumeric figures mean and are unsure which variant is better and suitable for their cars’ engines. To better understand what makes 10W-30 different or similar to 10W-40, we must first point out what the W stands for, and at the same time, what those digits indicate.

Winter and Summer Grade Oil Viscosities

The Society of Automotive Engineers (SAE) designed a scale that classifies engine oil through its viscosity. The SAE has come up with several categories, two of which are the 10W-30 and the 10W-40.

If you look at it one way, you will notice that the values—10W, 30, and 40—are all single-grade engine oil viscosities. That’s because multi-grade engine oil, the ones present in the market today, was not invented until the 1950s.    

While many people mistake the W for weight, it generally means winter. In this case, 10W indicates that both oil types work well in temperatures ranging from -30° to 0° C (22° to 32° F). That means the motor oil has low viscosity and remains fluid even in freezing temperatures, which allows the oil to lubricate the engine properly. If the oil is too thick, however, you may experience startup problems as well as reduced engine efficiency.  

Conversely, the two remaining values, which, in this case, are 30 and 40, indicate the engine oil viscosity when operating in hot temperatures. According to the SAE, 30 single-grade motor oils are suitable for engines that run in temperatures ranging from 0° to 30° C (32° to 86° F). On the other hand, the 40 engine oil will do well in higher temperatures that range from 10° to 40° C (50° to 104° F).

Differentiating 10W-30 Between 10W-40

Now that we have determined the values and what they represent, we can tell what makes the 10W-30 different from the 10W-40. However, we have yet to differentiate both oil types concerning engine performance and which oil type is the better option in any given circumstance. For that matter, the different factors that will help set both oil types apart from one another are as follows:  

Running and Operating Temperature

As explained above, both the 10W-30 and 10W-40 motor oils have the same low-temperature viscosity. This means both varieties have the same thin viscosity which makes them suitable for cold engine startups.

When it comes to practical applications, if you live in a country that constantly experiences cold weather, 10W-30 is a better option because of its lower viscosity level. Since the oil retains its fluidity despite the low temperature, it lubricates the engine well, allowing it to run effectively and smoothly.

On the other hand, if you live in a warm country, 10W-40 is obviously the more suitable motor oil for your engine. Because it is thicker than the 10W-30 variant, this motor oil does not thin out even when subjected to high temperatures.


High-mileage cars, particularly the ones that have run for 75,000 miles, would usually require thicker engine oil. That is because, over time, older engines get worn down from friction, so some of their moving parts become looser. In this case, 10W-40 is the thicker motor oil.

Therefore, if you have an older car that normally uses oil with lower viscosity, such as the 10W-30, for instance, switching to 10W-40 would be a clever decision since it would beneficial to your engine.

However, this is not always the case. With the technological improvements in lubrication engineering, oil manufacturers have come up with better oil alternatives for such vehicles, and one of which is high mileage oil. In that case, instead of upgrading from 10W-30 to 10W-40, choosing 10W-30 high mileage oil would be a smarter and better selection for your car.

Engine Clearance

As a rule of thumb, engine oil with low viscosity is far more suitable for engines with a tighter bearing clearance. Alternatively, thicker motor oil, such as the 10W-40, works well for vehicles with loose engine clearance because the high viscosity level of the oil enables it to fill up the minute gaps and spaces between each moving part. That is why 10W-40 works well for older engines.

Fuel Efficiency

In hindsight, thin motor oil is a better choice because it has minimal drag as compared to the one with a thicker viscosity, while it also reduces friction and metal-to-metal contact at the same time. Moreover, your vehicle will not require much effort in pumping the oil into the valve train to lubricate every moving component inside the engine.

However, experts do not advise switching from 10W-40 to 10W-30 for fuel economy. Instead, you should choose the correct engine oil that is suitable for your driving and engine condition.

You will be wise not to compromise the lifecycle of your engine and all the components inside of it simply because you wanted to save up lessen fuel consumption. In that case, always use the type of oil specified by the manufacturer.


When it comes to versatility, selecting the engine oil that performs gives you the best of both worlds is a sounder option. If we are to make a standard choice for light vehicles, though, the 10W-30 would certainly fit the bill. That’s because not only does it have the capacity to prevent cold starts, but this oil variant works nearly as well as the 10W-40 if we consider its operating temperature. Apart from that, 10W-30 engine oil is easy to come by as they are cost-effective.

Heavy-Duty Purposes

By heavy-duty applications we mean vehicles that are always pushed to their limits. Typically, high-performance engines get subjected to extreme pressure and high temperature. Thicker motor oil will constantly lubricate every mechanical component inside your engine, even if the temperature soars up to 30° C (86° F). At that rate, oil with thin viscosity, like the 10W-30, will not hold up. It would be too thin to provide the lubrication the engine needs to prevent metal-to-metal friction. For that matter, 10W-40 is a wise choice.

Final Takeaway — Which Oil to Choose?

So, which is the better option, you ask? Which should you choose for your car—the 10W-30 and the 10W-40? It all depends on the condition of the engine and the changes in ambient and engine temperature. Typically, both variants work as expected under regular weather conditions. Additionally, both motor oils lubricate well and have an exceptional capacity in reducing engine wear and corrosion.

However, if you consider other variables, such as mileage, utilisation, and engine type or composition, one oil type becomes a better option for your car. Regardless of which motor oil you choose, what matters is that you follow the manufacturer’s specifications and that it helps extend the lifespan of your engine.