30/40 son especificaciones de viscosidad de los aceites de motor.

Grades of motor oil
Single-grade motor oil
The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. See the article on viscosity for details on kinematic viscosity. For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100°C (212°F) in units of centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as an SAE number 0, 5, 10, 20, 30, 40, 50, or 60. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the weight of a motor oil. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.

The viscosity of single-grade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases logarithmically in a relatively predictable manner. On single-grade oils, viscosity testing can be done at cold winter (W) temperature (as well as checking minimum viscosity at 100°C or 212°F) to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W. A single-grade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature; i. e. a 10 grade oil should correspond to a 10W oil. For some applications, such as when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines.


saludos

In addition to the viscosity of oil, it carries an API designation starting with SA, SB ..... to SH. These letters from SB on indicate oil that contains additives to combat heat and contamination from internal combustion engines. (The API designation can be seen on a can or bottle of oil)

For lubrication uses in equipment and machinery, the additives are not desirable, so regular oil would have no additives and carry the SA designation.