10W30 vs 10W40 Oil: What’s the Difference & Which Is Better?

In a previous article, we covered low-temperature grade motor oils 0W-20 and 5W-20. Now, we’ll go over the differences between 10W30 and 10W40. Both oils are versatile multi-grade oil types and perform well in cold and standard weather. They may seem alike – but one is, in fact, used over the other for specific conditions and ambient temperatures. What’s the difference between 10W30 and 10W40 motor oils, and which is better?

10W30 and 10W40 engine oils are very similar. While both are low-temperature grade oils, 10W40 is thicker at high temperature; therefore, better to use in warm weather.

This article provides tips for using SAE 10W-30 and 10W-40 and covers viscosity grades. It will also go over the benefits of using thicker oil versus thinner ones for vintage engines and the right circumstances for utilizing each oil type. Read on to determine which is better between these two motor oil variants.

Motor Oil Refill with Yellow Funnel

Oil Viscosity and Temperature

Oil viscosity pertains to a liquid’s internal friction or ability to flow at specific temperatures. Understanding this is crucial as not all motor oils have the same resistance to flow. Colder weather conditions require (winter-grade) thinner oils to reduce friction and make cold-starting easier. Conversely, warmer temperatures call for (summer-grade) thicker oils, which have better film strength and sealing properties due to their higher viscosity.

The SAE J300 EOVC is the best reference to distinguishing winter from summer-grade engine oils. It grades motor oils according to their viscosity characteristics. A total of 14 high- and low-temperature viscosity grades cover both mono-grade and multi-grade oil types. The format for viscosity classification consists of 2-3 alphanumeric codes for mono- or single-grades and XW-XX for multi-grades – the latter showing a combination of cold-operating and hot-operating viscosity ratings. For instance, SAE 10W-30 oil would act like an SAE 10 at cold temperatures and like an SAE 30 at 100 °C (212 °F).

10W30 and 10W40 Defined

SAE 10W-30 motor oil is one of the low-temperature multi-grade oils commonly recommended by manufacturers, along with 15W-40, 5W-30, and 0W-20. This engine oil acts as an SAE 10 in sub-zero weather, yet functions as an SAE 30 in standard operating temperature. This oil type will not have problems in freezing climates up to -30°C/-22°F. Cold-starting should be hassle-free, even if you leave your wheeler outside of the garage.

Similarly, SAE 10W-40 motor oil performs well in cold weather conditions. It has the same low-temperature limits as the 10W-30 but is better in higher temperatures. This oil type remains thicker at 100°C/212°F compared to an SAE 30 high-temp flow grade and is more suitable for warm weather. Because this oil is more glutinous, it is better at resisting thermal breakdown and reducing deposit formation.

Both 10W-30 and 10W-40 are low-viscosity oil grades that contain polymers, which speed up or slow down the engine oil’s thickening or thinning rate, depending on oil temperature fluctuations. They are both available in pure conventional form or as high-quality synthetic grades. Either 10W30 or 10W40 for summer use is fine; however, 10W-40 motor oil will better protect your engine.

Proper Use of Motor Oil 10W30 vs 10W40

Adding Oil to the Vehicle Engine

When deciding to use one motor oil grade over the other, consider the following factors:

Ambient Temperature

Although environmental temperature does not add to the heat your engine generates during operation, it impacts oil viscosity. This is the reason your location is so important in selecting which oil to use. Between a 10W-30 and 10W-40, 10W30 motor oil would run smoother in colder climates while a 10W40 would be more effective in preventing engine wear and tear in warmer temperatures.

Fuel Economy

10W-30 oil tends to be more common and widely available, making it more inexpensive to use. Because it takes less energy for the engine to pump it, many vehicle owners prefer it over the 10W-40. However, it is unwise to use this oil type for gas mileage if it is not what the manufacturer recommends. Consider the potential trade-off between engine longevity and fuel cost-effectiveness when going for the cheaper option.

Manufacturer Specifications

For adequate lubrication of your engine’s internal components, it is best to refer to the rating specified by the manufacturer. Some, if not most, vehicle designers recommend more than one viscosity grade, giving you the option to choose what is most suited to your riding conditions.

Motor Oil Blend

Despite being close to negligible, 10W-30 and 10W-40 engine oils still have performance differences. This is especially true between conventional and synthetic motor oil variants. Synthetic blends provide more effective engine protection and temperature stability compared to conventional ones. That said, a synthetic 10W-30 may beat a conventional 10W-40 in protecting your ATV’s bearings and piston skirts.

Additionally, choosing between a standard and 10W30 vs 10W40 high mileage blends depends on how old your vehicle’s engine is. Using more viscous oil for older power mills believes that it will prevent the wear of oil passages is incorrect. Modern engines (produced in the last decade) do not benefit from higher-viscosity oil as it only adds stress to the oil pump. Choosing a high-mileage variant provides vintage engines adequate lubrication while dealing with aging-seal problems.

Starburst Certification

Go for oil labels with the right viscosity grade and display both the API donut and the Starburst symbol. The latest API service standards include SJ, SL, SM, SN, and SP for gasoline engines and CH-4, CI-4, CJ-4, CK-4, and FA-4 for diesel ones. The starburst symbol indicates the oil has passed SL service tests and that the American Petroleum Institute has verified it. Some manufacturers may also recommend motor oils with ACEA and JASO labels.

Viscosity

This aspect is vital when choosing between 10W30 and 10W40 motor oils (and is the focal point of this read). While both winter-grade oils perform similarly and reduce drag during cold startups, the 10W-40 motor oil is more viscous in hotter climates and has a better temperature spectrum. 10W-30 synthetic oil’s range is between -25°C/-13°F to 30°C/86°F (sometimes -30°C/-22°F to 35°C/95°F) while a 10W-40 is from -30°C/-22°F to 40°C/104°F. Inside your engine, both oil formulations can withstand temperatures of up to 100°C/212°F. But you know which oil type will work better.

Your Owner’s Manual

Referring to your service manual is crucial. This reference tells you which lubricant to buy and specifies the level of quality testing the oil should have passed. If you’re a new vehicle owner, you should follow the manual’s recommendation.

Checking Vehicle Oil Level

Mixing 10W30 with 10W40

You may get different responses when asked about mixing these two oil types. Some say that mixing them is fine as long as you don’t live in freezing weather. Even if you put together synthetic and conventional blends of these motor oils, they show no harm to your vehicle. However, this is true only if you mix in a quart of the other type to your usual engine oil.

Most mechanics and vehicle owners avoid mixing 10W-30 with 10W-40 oil for several reasons. One is to prevent a decrease in oil pressure when cornering at high revs, resulting in spinning a bearing or two.

Bear in mind that mixing motor oils may affect your vehicle’s warranty and fuel consumption. Although both 10W-30 and 10W-40 lubricants function the same during cold startups, one is slightly thicker than the other and would be less practical for certain high ambient temperatures. Unless you are in an emergency where combining them is the only option left, avoid mixing the two oil grades. Also, always have spare motor oil with you when you travel to avoid having to mix.

Motor Oil Blends

Here are some tips on which oil mixture, 10W30 vs 10W40, to choose, depending on your engine makeup and driving style:

  • Premium Conventional Oil. Conventional oils are becoming less of an option for off-roaders, as this mineral oil type is designed specifically for simple engines and riders with regular driving styles. Should you fall under this driver category, you are better off with a conventional oil brand with the service level SM or higher. Otherwise, look to the other oil blends if you prefer aggressive riding.
  • Full Synthetic Oil. Because this oil blend has fewer impurities and better extreme-temperature performance, full synthetics are usually compatible with high-tech engines found in race cars and come with a heftier price tag. These oils are chemically engineered and not meant for all engine types. Both 10W-30 and 10W-40 rarely have full synthetic variants, even for diesel oils. Valvoline Advanced Full Synthetic SAE 10W-30 Motor Oil (view on Amazon) is a good example of this blend.
  • Synthetic Blend Oil. This blend is the most common variant for both 10W-30 and 10W-40 engine oils next to conventional oil types. This blend is mixed with organic oil, which aids in resistance to oxidation. It is the most economical of the oil blends and is excellent for hauling and off-pavement riding. Going for brands like Liqui Moly 2043-4PK MoS2 Anti-Friction 10W-40 Motor Oil (view on Amazon) that you can buy packs of four will help you save on costs down the road.
  • High-Mileage Oil. High-mileage oils cater to vehicles with engines older than a decade and six-figure miles on the trip odometer. Its seal conditioners help with aging-seal problems, helping restore seal flexibility and covering piston-to-cylinder clearances better. This formulation also has anti-wear additives that slow down engine wear. It is perfect for riders who still believe in the might of their vintage wheelers.

10W30 vs 10W40 Oil FAQs

  • What is 10W 30 oil recommended for? 10W-30 motor oil is recommended for ambient temperatures not exceeding -30°C/-22°F in the winter or 35°C/95°F in the summer. This is a great motor oil to use if you live in Canada or cooler U.S. states. Colder regions would require 5W-20, while hotter regions may call for 10W-40.
  • Is 10W40 good for winter?  Generally, the answer to this is yes, since it is a low-temperature oil grade. However, there will be certain exemptions. For instance, 10W-40 is not be ideal for people living in Nunavut or North Dakota than those living in other American states or Canadian townships. It was also observed that 10W-40 does not have cranking numbers as good as 10W-30 engine oil.
  • Is 10W40 synthetic? 10W-40 engine oils are mostly synthetic blends. Their base is refined crude oil with additives. It will not be long before full-synthetic variants become the standard, as the market is now moving towards that direction.
  • 10w30 vs 10w40 which is better? It depends. With changing ambient temperatures and different driving behaviors considered, either oil type will be suitable. Take your vehicle’s current state into consideration and refer to your owner’s manual for guidance.

Conclusion – 10W30 vs 10W40 Oil: What’s the Difference & Which Is Better?

There is little difference between these two engine oil types. What separates them apart has more to do with temperature fluctuations, engine condition, and consumer preferences. For average weather conditions, either motor oil is an excellent option. But depending on engine makeup, budget for fuel, vehicle use, and riding style, one may outperform the other. Nevertheless, both engine oils are efficient in providing adequate lubrication, shielding your engine against corrosion, and helping in the safe-keep and maintenance of your wheeler.

Kris Peter

Adventure seeker and off-road enthusiast. I love the thrill of going off-road and taking on the elements.

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