What do the numbers on motor oils mean?

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The SAE (Society of Automotive Engineers) Viscosity Index is a classification used to categorize motor oils according to their viscosity grades at different temperatures. Viscosity is defined as a liquid’s resistance to flow; in oils, this influences their ability to protect cylinder walls and other components such as the crankshaft, connecting rods, camshaft, etc.

SAE Viscosity Index

The SAE index is represented by two numbers (for example: 10W-40), in which the first number, which is accompanied by the letter “W” (which stands for Winter) indicates the viscosity grade of the oil at low temperatures, that is, the number indicates the oil’s ability to flow when the engine is cold.

The second number after the dash indicates the oil’s viscosity grade at high temperatures (100°C). An oil with a higher number after the dash means it will have a higher viscosity at higher temperatures, which broadens its optimal operating range to higher temperatures.

Multigrade oils

Multigrade oils have the versatility to change their viscosity under different thermal conditions. At low temperatures, they flow more easily to more efficiently lubricate each engine component. At higher temperatures, viscosity increases, increasing the resistance to flow, forming a protective layer on engine components that need to be protected from metal-to-metal friction. This is why the SAE rating, when accompanied by a number with a “W” followed by a hyphen and two digits, means the oil is multigrade.