With the name of the oils you identify a class of liquid mixtures used for the lubrication of the mechanical parts.
From the chemical point of view, they consist of a base nature, hydrocarbon and a combination of additives (typically from 10 to 30% in volume), which may be of mineral origin, semi-synthetic or synthetic. In the case of the lubricating oil used in vehicles, it speaks more specifically of engine oil.
synthetic: additives obtained by chemical processes laboratory, ensuring superior performance in extreme conditions of temperature, pressure, and stress;
semi-synthetic additives mixed obtained by chemical processes or present in nature;
minerals: additives that are not generated by the chemical processes of the laboratory.
The code can be constituted by a single number ("oil monograde oil"), which describes the viscosity of the liquid at a specific temperature (in the case of the ISO grades VG, which are part of the classification ISO 3448, 40 °C), or may be constituted by two numbers ("multigrade oil") where the first number is followed by a W (gradation Winter) and determines the value in the condition of low temperatures, while the second determines the value in the condition of high temperature.
The lubricants are multigrade (SAE 5 W-50 SAE 10 W-40, etc.,) are: Oil can maintain the performance under different temperature. These oils are classified according to a code composed of two numbers separated by a W ("winter"); the first figure indicates the viscosity of the oil at lower temperatures; the second digit, the most high, indicates the viscosity at high temperatures.