What do the numbers in the oil viscosity designation mean?

The viscosity designation according to the SAE (Society of Automotive Engineers) standard always consists of two parts separated by the letter “W” (Winter – winter):

The first digit (with the letter W). Indicates low-temperature viscosity. The lower this figure, the better the oil behaves at low temperatures. For example, oil with the designation 0W will be more fluid in the cold than 10W oil. This is crucial to facilitate engine start-up in the cold season, as oil with a low low-temperature viscosity is pumped faster through the lubrication system, providing protection from the very first revolutions.

The second digit (without the letter W). Indicates high temperature viscosity. The higher this figure, the more viscous (thick) the oil remains at high engine operating temperatures. This means that it will retain its oil film and anti-wear properties even under extreme heat. For example, 10W-40 oil is more viscous at high temperatures than 5W-30.