Which of the following describes demulsibility in lubricating oils?

Demulsibility in lubricating oils specifically refers to the ability of the oil to separate cleanly from water. When lubricating oils are exposed to water, either from leakage or condensation, a demulsible oil will allow the water to separate out without forming an emulsion. This separation is crucial because emulsified water can lead to corrosion, reduced lubrication performance, and formation of sludge. Therefore, a lubricant that demonstrates good demulsibility ensures that any unwanted water is removed from the oil system, thus maintaining optimal lubrication properties and prolonging the life of the machinery.

The other options relate to different characteristics of lubricating oils: providing a consistent film thickness and lubricating metal surfaces pertain to how well the oil performs under load and how effectively it reduces friction, while resistance to temperature changes describes the thermal stability of the oil. While these features are important, they do not directly define demulsibility.