Today, the engine oil market offers a wide range of products for cars of various makes and models. However, with such a variety, one-on-one car owners face a difficult choice problem, trying to focus not only on the quality of motor oils, but also on their viscosity, seasonality, compatibility, and also the manufacturer. At the same time, according to statistics, the most popular automotive oils, regardless of brand, include liquids marked 5w-30 and 5w-40. But despite the popularity of these products in automotive circles, few drivers know how these fluids differ from each other. Today we will figure out what is the difference between 5w30 and 5w40, as well as which one is better to choose.

The main criterion by which these two types of engine oils can be distinguished is the oil viscosity (SAE). It is viscosity that is one of the main indicators of the performance of engine oil. According to the SAE classification, the first digit of the marking (5W30, 5W40) is an indicator of the viscosity of the oil in the cold season, and the second (5W30 , 5W40 ) in the warm season. * The Latin letter “W” is an abbreviation – Winter.

Let’s figure out the differences between oils marked SAE 5W-30 and 5W-40 on the canister, which one to choose, whether it is possible to replace one with another. I continue the series of articles comparing oils of different viscosities.

Any difference between SAE oils is in viscosity. In this case, the low-temperature indicator is the same, but the high-temperature parameter is not, which means there will be a difference in viscosity. 5W-30 has a kinematic viscosity at 100℃ in the range of 9.3-12.5 mm2/s, 5W-40 – 12.5-16.3.

It should be understood that although this classification is perceived as a kind of climatic indicator for the use of oil, in general it is just a viscosity, that is, SAE shows how easily the oil will be pumped through the channels, how thick the oil film forms on the parts, at what temperature below zero the motor will start safely and much more. This characteristic is very important due to the difference in the parameters of different engines, for example, some models have a very thin and long oil channel, through which the pump simply cannot pump viscous oil. Therefore, it is absolutely impossible to compare oils according to SAE in the context of which one is better.

Often oils with such a viscosity are synthetics, semi-synthetics are less common. To more clearly show the difference between the indicated viscosities, we will take synthetic oils as the basis for the discussion.

Comparison of laboratory tests 5W-30 and 5W-40

For example, let’s take the synthetic oils of the domestic manufacturer Lukoil Lux:

From the comparison table, the difference between the oils is obvious, which, in principle, will be for any lubricants of the discussed viscosity, if we compare them with each other. Viscosity characteristics at 40℃ and at operating temperature differ dramatically. There are no temperature readings. And it goes without saying that the thicker 5W-40 showed more dynamic viscosity. So, if you choose oil for the winter and the choice is between these two SAEs, then of course it is better to stop at 5W-30, but after studying the technical characteristics of the brand that you choose. Only the SAE indication is not the ultimate truth, the indicators may vary within the limits allowed by the classifications.

5W-30 or 5W-40 – which is better?

As I said above, these oils cannot be considered from this point of view – this is better, and this is worse. We are guided by the recommendations of the engine manufacturer, what viscosity is indicated there, we pour it. Usually two numbers are indicated, between which you can choose a less viscous one for winter or a more viscous one, if necessary.

What else needs to be understood, I repeat, SAE is not an exact standard and allows indicators to float within the established limits, this is especially noticeable in the example of low-temperature characteristics. Much depends on the composition of the oil. For example, 5W-40 synthetics may have a lower freezing point than another manufacturer’s oil made using a different technology, but with the same SAE viscosity.

In some cases, drivers increase the viscosity of the oil in order to reduce its consumption – this can be done, but as a temporary measure, in the end, you still need to figure out what is the reason for the increased consumption, and mentally prepare to capitalize the engine if it has already worked out the assigned mileage resource.

Which oil is better in winter – 5W-30 or 5W-40

In a specific example with Lukoil Lux, which I compared a little earlier, 30ka would be the best choice due to the lower dynamic viscosity. The freezing point of the oils are similar. But at the same time, an oil with a higher “summer” index will better resist flow when it reaches operating temperature due to a higher kinematic viscosity at 100℃. This is the comparison you need to make when choosing oil for the winter – do not rely only on SAE, but compare all technical specifications.

Can 5W-30 and 5W-40 be mixed?

You can mix, but in rare cases, using the same brand of oil and not abusing such mixing. In this case, it must be understood that the viscosity of the oil will drop or rise, depending on the amount of oil added. That is, if you mix the indicated viscosities 50 by 50, the result will be an average value of 5W-35. We also take into account the composition of the oil – synthetics, semi-synthetics or mineral water. I have already considered mixing oils in more detail earlier in another article.