10W30 vs. 10W40 Engine Oil: Which is Suitable for You?

Jerry Wilson
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What Do the Numbers Mean?

Car owners are all familiar with the oil specification information that’s printed on engine oil: 5W30, 10W30, 10W40. At first glance, they seem to tell us the percentage of viscosity that other engine oils are thinner or thicker than each one.

So 10W40 sounds thicker, and 5W30 sounds thinner.

Each number represents the viscosity specification index. If you’re not familiar with viscosity index, it’s a measure of how far a liquid will flow before it changes form.

The higher the number, the more viscous the liquid is … for example, honey has a higher viscosity index than water. This number is determined by measuring a given liquid’s viscosity at its boiling point, and then again at 30F.

For example, if you take a jelly, it will take a much longer time to travel two feet than it will take maple syrup to travel two feet, even though both have the same viscosity index.

When to Use Which

Engine Oil?

Periodic engine oil changing is essential in avoiding decline in engine performance and efficiency. Not only that, in the long run, it can be very costly if you neglect the regular replacement of your engine oil. You may need to extend your car's engine for another 150,000 miles if you follow these simple rules of thumb when it comes to choosing the best oil for your engine.

What is engine oil?

Engine oil serves as the coating or lubricant between metal parts inside the engine. As the engine parts rotate against each other, the engine oil's primary responsibility is to reduce heat and wear and resist damage from these parts. Oil also serves as a coolant for the engine and keeps the engine parts free from contaminants.

Engine oil is manufactured to have appropriate load and viscosity characteristics based on what type of engine you have. An engine that is designed to be powerful and produce high-speed output usually requires oil with a thicker viscosity, while an engine with a lower output running a vehicle under mild conditions is best served by lighter engine oil.

Engine Oil Ratings and Recommendations

Engine oils are commonly rated using the American Petroleum Institute formulation of rating, in which the numbers correspond to different criteria for viscosity. The following is a commonly used rating scheme for engine oils:

10W-40: Ideal for stock engines. Manufacturers usually recommend the use of 10W-40 for stock vehicles.

Fuel Economy

10W30 is a thinner oil, and it will flow better in cold weather than a 10W40 oil. This means your car will start easier in the winter, but it can also mean more oil use and a shorter oil life. So if you live in colder climates, you should probably stick to the 10W40.

If you want to minimize the oil changes for your vehicle, you should consider using the 10W30 engine oil because it will encourage manufacturers to make oil filters that last longer.

So which oil should you choose?

As you can see, both oils can deliver the same level of protection for your engine. When choosing between the two, it’s best to go with the one that benefits your specific driving situation the most. Remember, you should always check your manufacturer recommendations before making your choice.

The same goes for mixing oil. The oil life of 10W30 blends is no shorter than that of pure 10W30 oils, so you may consider mixing the two grades in order to get the best protection at the lowest price.

Conclusion

Choosing the right oil is not easy because in most cars, oil weight choices are not clearly marked. If you're like most, you may have already switched from the standard grade 30 after being told that it’s not good enough for your car.

If you are also in this category, you may be left wondering what should you now use. The good news is that there's a change, and this one will make you happy, because race car drivers already know it. For lubrication, it is now increasingly common to reach for the 20W-50 instead. And even if your car was made years after 1950, the rule is you always have the freedom to appoint whatever oil you deem fit.

As a rule of thumb, if the temperature in your climate frequently dips below +5 degrees Celsius (about +40F), you should be using synthetic engine oil. Your engine requires this regardless of the car you are using.

Any synthetic oil as well as the brand you choose should have the following: American Petroleum Institute (API) certification, adhering to the SAE standards, 7th edition and later.

There are some other factors to consider, though. Your car's make and model are the number one factors that determine the kind of oil you should invest in. You can also use a car oil calculator that assists you along the way.