What do the oil viscosity labels (5W-30, 10W-40) mean and how do they relate to the season?

SAE (Society of Automotive Engineers) viscosity designations are the key to understanding the seasonal characteristics of an oil.:

The first digit with the letter “W” characterizes the winter viscosity of the oil. The lower this figure, the more liquid the oil remains at low temperatures. This is crucial for starting the engine in the cold, as the more liquid oil is pumped faster through the lubrication system, protecting the parts from the very first revolutions. Oil marked 0W will be the most fluid in the cold, followed by 5W, 10W, and so on.

The second figure characterizes the high-temperature viscosity of the oil, that is, its “density” at the operating temperature of the engine (about 100 ° C). The higher this figure, the more viscous the oil remains when heated. This means that it will better retain its protective film between the engine parts under high loads and temperatures.

Thus, the designation of viscosity in itself indicates the suitability of the oil for various temperature conditions throughout the year.