What does 10w/40 written on the front of the engine oil mean?

[DiGabrielle]

The 10W-40 marked on the bottle represents the viscosity grade of the lubricating oil product (indicating better fluidity at low temperatures and better viscosity at high temperatures). For detailed information, please see below: 1. The American Petroleum Institute classifies engine oils into different grades based on their quality levels: This standard uses the letter "S" to represent gasoline engine oils, and different grades are arranged in alphabetical order after the letter "S", such as SE, SF, SG, SH, SJ. The letters following "S" indicate higher oil grades as they appear later in the alphabet. 2. Classification of oil quality by viscosity grade: (1) The first category is single-grade viscosity, which is further divided into high-temperature and low-temperature types. (2) The second category is multi-grade viscosity all-weather type: Oil grades include SAE5W-30, SAE15W-40, SAE20W-50. The first part before the hyphen in this grade indicates the oil's low-temperature viscosity performance in winter, and the second part after the hyphen indicates its high-temperature viscosity performance in summer.