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Are “thin” oils good for your engine?

Are you familiar with the statement “I’d rather use thick oils because when using thin ones, consumption is higher, and they don’t lubricate well enough in the summer”? The question whether there is any practical or scientific basis for such misconception encouraged us to explore this topic together with our expert associate, Dr Stevan Dimitrijevic, to give a detailed answer.

Which oils are considered “thin”?

Strictly speaking, a definition of engine oils popularly referred to as “thin” does not exist. One could say, in a simplified manner, that these are oils with lower viscosity than what is considered “usual”.

  • During the 80s when SAE 15W-40 was the most common viscosity grade, “thin” oils were SAE 5W-40.
  • In the 90s, apart from the standard SAE 10W-40 these were mainly SAE 5W-30 and SAE 0W-40.
  • In the first decade of the 2000s SAE 5W-30 became standard (in Euro 4 and especially Euro 5 engines), and SAE 5W-40 a maximum viscosity grade recommended as default. Since then, many consider SAE 0W-20/30, as well as low HTHS 5W-30 (below 3,5 mPa⋅s) as “thin” oils.

HTHS (High Temperature High Shear) signifies viscosity under precisely defined conditions meaning high temperature and high shear rate. 

Prejudice about engine wear and higher consumption

Ever since these oils, primarily SAE 5W-30, ACEA A1/B1, A5/B5 and ACEA C1/C2, became an official recommendation (Ford introduced them at the end of the 20th century) there has been a fear that such “thin” oils would not provide enough protection against engine wear

Although it is true that higher viscosity oils have a thicker oil film, although measured in micrometers, engine protection is essentially related to the bearing clearances, and engine clearances in general. Therefore, modern engines require lower viscosity oils to achieve the same effect. This should be always kept in mind. 

The more contemporary (new) engine construction, the lower the tolerances and the smaller the clearances, with rare exceptions.

Therefore, it is best to follow the engine manufacturers’ recommendations however odd they may seem, like for example with SAE 0W-30 for Japanese diesel and SAE 0W-20 for petrol engines, even dating from the beginning of this century.

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Be that as it may, doubts on the issue still arise, so let us consider what causes them:

1) Some vehicle manufacturers allow a much higher viscosity as alternative, most often 5W-40 along with 5W-30, but also 5W-40 along with 0W-20, or rarely 10W-60 along with 5W-30.

2) Some older generation mechanics are still doubtful about lower viscosity oils. Why? Because such oils did not exist in their time, so mistrust is somewhat expected to a certain extent. Additionally, engine wear is also taken into consideration, which leads us to the next point.

3) Many miles driven make bigger clearances inside the engine, which provides at least some kind of basis for considering higher viscosity oil. We will discuss this point a little bit further in this text.

4) It often happens that engines consume lower viscosity oils more than higher viscosity oils. However, bear in mind that this is mainly true for older vehicles, with worn engines and/or capillary seal leakage.

Namely, using higher viscosity oils than what is recommended, or within the manufacturer’s recommendation, can compensate engine wear to a certain extent. However, one needs to bear in mind the following: 

As long as the engine performs within factory designated tolerances, or simply put “the engine is in good condition”, the default viscosity grade recommended by the vehicle (engine) manufacturer is certainly enough for ideal engine protection. 

If the vehicle manufacturer allows higher viscosity oils (e.g. apart from 5W-30 they allow 5W-40, or instead of 0W-30 they allow 5W-30) you may switch to the higher viscosity grade if the vehicle is  older than 7 or 8 years, or it has been driven for over 200.000 km. This is, of course, not mandatory, but it is grounded to a certain point, because the engine is at least slightly worn. Switching from 5W-30 to 10W-40 (allowed for some Japanese models) means only lower maintenance costs. However, if the engine is in good condition, you do not need to do that even after 200,000 km.

Only if oil consumption is somewhat higher, e.g. from initial 0,5 liters at 10.000 km, now it is 1liter or slightly more - and there are no leaks on the seals, then you can switch to a higher viscosity oil. This is virtually the only and somewhat justifiable case, as the engine is not yet ready for repair and can be used for another 100,00 km or even more. Of course, this is valid only up to the moment when oil consumption begins rising abruptly, multiplying the initial oil consumption, and only until the engine is able to pass the emission test, because if not, it will need to be repaired.

If you are NOT SURE, check now and see if your engine consumes TOO MUCH oil.

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Prejudice about the quality of lubrication during summer

Lower viscosity oils are intended for newer engine constructions and usually cover all climate zones, at least those in Europe. They are particularly suitable for the continental climate zone (northern Europe) and generally, places with strong winters where extremely low temperatures are common.

Let us remind ourselves: SAE 5W-30 oils are suitable for temperatures down to -25 °C, while 0W-20/30/40 are suitable down to -30 °C. Of course, many modern oils can go 5 to 10 degrees below that, and a good reference point would be 10 degrees above the pour point. This means that if, for example, an oil’s pour point is at -45 °C, it can be used at temperatures down to -35 °C.

We will illustrate this using the following example. According to the ASTM D97 test, Total Quartz INEO First SAE 0W-30 has a pour point at -48 °C, which means it can be used at temperatures down to -38 °C. We need the mention also that the validation test shows -40 °C so the oil is reliable for cold starts even at that temperature. This makes it suitable for the most part of the planet, even for areas like Siberia, Alaska, northern Canada and of course, northern parts of Scandinavia. 

However, what poses an even greater question is whether low viscosity oils are suitable for high ambient temperatures. 

For engines they are made for – YES, definitely!

A few days, even around 10, when outside is +45 °C in the shade and a few around +40 °C, should not dissuade you from using low viscosity oils.

Synthetic oils are definitely suitable for entire Europe, including the Mediterranean region. Therefore, the fear that such oils would not be suitable for Central and East Europe, even its southernmost part, is ungrounded.

Some vehicle manufacturers state that these oils can be used at +40 or even +45 °C, which guarantees usability in Greece, south of Italy and Spain. The Adriatic region, North Macedonia and especially countries to the north are completely compatible with these oils. 

For tropical and/or desert climate you should consult the vehicle manufacturer or their authorized service to check if you might need a higher viscosity oil. 

Conclusion

  • If your engine is designed to be lubricated with the so-called “thin” oils, do not hesitate to use them at least up to 200,000 km. 
  • After that you could switch to an alternative recommendation by the vehicle manufacturer (e.g. from 0W-30 to 5W-30 with the same or similar specification), but this is certainly not mandatory.
  • The cost of changing oil is not so high as to make some significant savings, even when taking into consideration changing the oil filters and the service fee. So, don’t do it only for the sake of saving money or “just in case”, as long as your engine is in good condition. One viable reason for such a thing could be an easier purchase.
  • What is more, avoid switching to a higher viscosity oil if the vehicle manufacturer does not state one in its recommendations. Not for the vehicle’s age (you can simply change the oil you have already been using more frequently) and especially not for a few hot days in the summer.
  • If the engine consumes some more oil than it used to at the beginning of its life cycle, switching to a slightly more viscous oil could be justifiable. Of course, only as long as the vehicle is able to pass the emission test and the consumption does not increase significantly, approaching the maximum stated in the owner’s instruction booklet. In case it does begin increasing significantly, do not wait for the maximum, but when it reaches half of the stated value (e.g. 0.3 dl from maximally allowed 0.6 dl at 1,000 km) consider repairing or replacing the engine or replacing the entire vehicle.

We hope this article which the Total team has prepared together with Dr Stevan Dimitrievic was useful to you. DON’T MISS more content recommended below, and feel free to contact us for any questions, doubts, and suggestions.