What Is the Viscosity Index of an Engine Oil?


    – Engine oil viscosity: the SAE classification

    – Criteria for choosing an engine oil according to its viscosity

    – Price of engine oil according to its viscosity index

If you change your engine oil yourself, the viscosity of your engine oil is one of the aspects you should be particularly careful about; otherwise, the manufacturer’s warranty and the integrity of your engine could be called into question… We tell you more in this article!

Engine oil viscosity: the SAE classification

Viscosity is the ability of an oil to flow.

The principle of measuring viscosity is to know the time and speed of oil flow for a given quantity and temperature.

The different types of oil

The viscosity of an oil is defined by the SAE classification (Society of Automatic Engineers). Let’s see closer below.

Monograde oils, which are further classified into two types:

  • SAE 5W, SAE 10W, and SAE 20W have low viscosity indexes and are very fluid and suitable for winter use (W = Winter).
  • SAE 30, SAE 40, SAE 50 with high viscosity, which is intended for warm seasons.

Multigrade oils

Today, the same oil is used throughout the year. The particularity of these multigrade oils is that they have 2 viscosity indexes: SAE 5W30, 10W40, 15W40, etc. are oils whose viscosity is stable over a more or less wide range of temperatures.

Oil compositions

Engine Oil

In terms of composition, the different types of oils are:

  • Mineral oils (produced from the distillation of petroleum).
  • Semi-synthetic oils (additives of chemical components).
  • Synthetic oils (chemically manufactured with additives, based on alcohol and petroleum).

Modern engines, which are highly efficient but demanding, require increasingly stringent specifications for oils, particularly for increasing the range of viscosity.

This is why the first generations of mineral oils (mono grades 15W40, 20W50) have gradually given way to semi-synthetic oils (10W40, 10W50) and then to synthetic oils (0W30, 5W30, 5W40…), the only ones recommended today for the latest generation of engines.

Criteria for choosing an engine oil according to its viscosity

Viscosity defines a type of oil and not a specific quality of oil; this is why it is necessary to be vigilant, when buying oil, about the manufacturer’s specifications, which require a viscosity that complies with the original and oil that meets the manufacturer’s internal approval and quality standards.

There are several ways to avoid making mistakes:

    – Rely on the maintenance booklet delivered by the manufacturer with the car, which indicates the appropriate oil;

    – Buy in a dealership, where you will deliver the oil corresponding to your vehicle to you;

    – Use online oil recommendation tools;

    – Beware of oil change packages from car centers with unbeatable prices: often, it will be low-end oil or oil that does not comply with the manufacturer’s recommendations.

Note: You may cancel the contract in an engine breakdown involving the warranty if the oil does not comply (after expertise and oil analysis).

Price of engine oil according to its viscosity index

Not surprisingly, oils with a limited viscosity index are the least expensive compared to the latest generation oils:

    – 1L of 10W40 multigrade oil costs $4 to $5;

    – 1L of 5W30 multigrade oil costs from $7 to $10;

    – 1L of 5W40 multigrade oil costs from $5 to $10;

    – 1L of 0W30 multigrade oil costs from $7 to $13.

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