Which property of oil is directly related to its ability to flow under pressure?

Viscosity is the property of oil that directly influences how it flows under pressure. It is a measure of a fluid's resistance to deformation or flow. When oil has a higher viscosity, it means that it is thicker and requires more force to flow. Conversely, oils with lower viscosity are thinner and flow more easily. This is particularly important in applications involving lubrication in engines and mechanical systems, where oil needs to circulate effectively under varying temperature and pressure conditions.

In contrast, lubricity refers to the oil's ability to reduce friction between surfaces in contact, which is related to its effectiveness as a lubricant but not directly its flow characteristics. The flash point is the temperature at which the oil can vaporize and ignite, and the pour point is the lowest temperature at which the oil will flow. While these properties are important for understanding the oil's behavior under various conditions, they do not directly address how well the oil flows when pressure is applied. Thus, viscosity is the critical factor in determining an oil's flow under pressure.