Different cars require different kinds of motor oil, which is why those numbers on the bottle are so important. Jason Fenske at Engineering Explained has a quick video explanation of what those numbers mean.
Note that the video is sponsored by Mobil1, though Jason uses the video to explain oil weights rather than extol the virtues of one brand of oil over another.
The numbers refer to what's known in the industry as the oil "weight," but that term is a bit misleading. Oil weight doesn't refer to how heavy an oil is; it's a measure of viscosity, or the thickness of a fluid. Syrup, for example, has a high viscosity, while water has a low viscosity. The higher the number on the bottle, the thicker the oil.
However, the numbers are also temperature dependent. A 10W-30 rating means an oil has a viscosity grade of 10 in cold temperatures ("W" stands for "winter"), and 30 in warm temperatures. That doesn't mean the oil gains viscosity as it heats up, though. It just means the oil will behave like a 10-weight when cold and a 30-weight when hot. In general, oil's viscosity actually decreases as it heats up.
Manufacturers typically uses additives to achieve temperature-specific viscosity ratings, and those ratings are certified with multiple tests.
Engineering Explained on conventional vs. synthetic motor oil
Cold testing usually ranges from -10 degrees Celsius to -40 degrees Celsius (14 degrees Fahrenheit to -40 degrees Fahrenheit), according to Fenske. Testers want to see if the oil will be thin enough for the engine to turn over, and to flow once the engine is running. At low temperatures, oil can thicken to the point that it actually solidifies.
Hot testing is done at temperatures between 100 degrees Celsius and 150 degrees Celsius (212 degrees Fahrenheit and 302 degrees Fahrenheit), Fenske said. In these tests, the focus is on seal leakage and the ability of the oil to protect internal engine components when it's in a thinner state.
You may have noticed the numbers on oil bottles getting lower recently, with some oils even wearing a 0W rating. That's largely down to efficiency, according to Fenske. Thinner oils produce less friction, meaning less energy is required for rotating engine parts to move. The potential efficiency improvements are small (often less than 1%, according to Fenske), but they still help automakers meet stricter fuel economy standards. Modern engines are designed to run with these thinner oils, so excess wear shouldn't an issue, Fenske noted.
Watch the full video for more details on how motor oil works. A lot of engineering goes into the numbers on each bottle of oil.