What it is and why it is key when choosing oil

We had already explained some time ago the keys to choosing the best oil for your engine. On this occasion we want to focus on a vital parameter when choosing it, the viscosity index of the oil.

This scale relates the temperature and viscosity of the oil. Viscosity takes into account resistance to gradual deformation. If we refer to car oil, then the viscosity index measures the ease with which that lubricant slides through the different parts of the engine, to prevent premature wear.

motor oil viscosity depends on temperature. The higher the heat, the less viscous the oil is (just like when you leave a bottle of oil or honey in the cold), which hardens and regains its characteristics when the temperature rises. The manufacturer designs its mechanics to work with a specific oil, which is why it is always said to use the oil recommended by the manufacturer.

To do this, each oil has a viscosity index, stipulated by the international classification of SAE (American Society of Automotive Engineers). This regulation specifies viscosity values ​​of different types of oils under certain thermal conditions. It is key to know them so that the engine operates safely.

To calculate it, a methodology was used analyzing two very different types of crude oil:

  • Pennsylvania Crude (paraffinic), used as a reference point at one end, since its viscosity hardly varies.
  • Texas Gulf Coast Crude Oil (naphthenic), which is just the opposite.

If a lubricant has identical reactions to Pennsylvania crude oil, a viscosity index of 100 is assigned. If, on the other hand, it resembles Texan crude oil, its index is 0. The higher the viscosity index, the more stable the viscosity is (which is search) in a greater range of temperatures. It is calculated between 40 and 100 ºC.

Types of motor oil

With this procedure the SAE establishes eleven types of motor oil, which range between 0W and 60. The W after the number indicates that it is an oil adapted to be used at low temperatures (for winter). We can divide them into two classes:

  • Six types of winter oil: 0W, 5W, 10W, 15W, 20W, 25W. It indicates the fluidity of the lubricant at subzero temperatures. The lower it is, the more fluid the oil will be in cold weather.
  • Five types of summer oil: 20, 30, 40, 50, 60.

You may have imagined that there are other variants, since you do not change the oil depending on the time of year. It is about the multigrade oils, suitable for use during 12 months of the year without fear of damaging the engine, which is what your car uses. They are named with two numbers and a letter W and are what drivers all over the planet use.

  • The first digit (joined to the letter W) shows the viscosity index at low temperatures. It is the minimum temperature at which a motor oil retains its viscosity in optimal conditions:

Viscosity index Temperature
0W -35ºC
5W -30ºC
10W -25ºC
15W -20ºC
20W -15ºC

  • The second figureshows the viscosity level at high temperatures, up to 100-150 °C.

There are all advantages with multigrade oils, since in addition to avoiding changing it depending on the time of year because they maintain the lubrication capacity at low and high temperatures, it is faster to start cold (avoiding engine wear) and it consumes less than others. lubricants. We remind you of another article on how to read an oil can, where there is much more interesting information in addition to the viscosity index.

There is also an important differentiation of oils according to their production process:

  • Mineral oils: they come from the distillation of petroleum in its pure state. They generally have a higher viscosity index.
  • Synthetic oils: Obtained directly in laboratories. They are the most common today, since they are capable of maintaining an adequate level in very different temperatures and offer better response in very extreme conditions… in addition to adapting better to the latest generation engines.

What if I use an oil with a different viscosity index?

In a multigrade oil you could lower the first figure to improve cold starting (the oil will reach all corners better) and can even reduce consumption. Never choose a higher primer figure as it may not flow fast enough at low temperatures. According to the second figure, it is not advisable to go up or down from the amount set by the manufacturer. If you lower it, the oil would be too thin when hot and the engine could have problems when maximum performance is required.

As you can see, if you use oils other than those recommended by the manufacturer of your vehicle, it will not stop working, but if there are very large variations with respect to the original, it can cause problems:

  • Viscosity index too low: the oil will become so liquid that it will drain without having generated the lubricating film between the engine parts to prevent their wear. You will not be able to avoid the loss of power either.
  • Viscosity index too high: The car will consume more fuel and the engine could overheat or lose some of its power if not enough lubricant flows through it, since the more viscous a lubricant is, the more difficult it will be to flow over a surface.

Can I mix two oils?

It is not advisable, always use oil with the viscosity index recommended by the manufacturer. If it is not indicated, the protective film that keeps the mechanics in good condition with use will not be optimally created. But in a moment of urgency (suddenly you see that your oil level is very low) you could mix different oils. Even a mineral one with a synthetic one. It will always be better than a car without oil.

Of course, remember that any mixture of lubricants of varied nature and with different properties reduces their quality and the viscosity would not be optimal. Travel as few kilometers as possible with that oil. Performing oil changes when indicated by the manufacturer is of vital importance, since over time it loses its properties (the sooner the worse the oil, since low-quality oils use more additives, which are the first to suffer over time… and viscosity varies.

In this case, the answer is very simple: the smartest thing is to follow the manufacturer's recommendations and always use the approved oil recommended by the car brand for your model. These same guidelines must also be followed in relation to oil changes, always trying to respect the frequency recommended by the manufacturer.

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