What characteristic of lubricants is important for their effectiveness under pressure?

Viscosity is the key characteristic of lubricants that determines their effectiveness under pressure. It refers to the thickness or internal friction of a fluid, which affects how well the lubricant can film over surfaces and reduce friction between moving parts. High viscosity lubricants can maintain their film strength even under heavy loads, preventing metal-to-metal contact that could lead to wear and failure of components.

Additionally, lubricants need to exhibit stable viscosity across a range of temperatures and pressures to ensure consistent protection and performance in various operating conditions. As pressure increases, a lubricant with appropriate viscosity will not be easily squeezed out from the contact surfaces, which is crucial for maintaining effective lubrication and preventing damage.

The other characteristics such as odor, color, and cost, while they may be of some interest in terms of safety, aesthetics, or budgeting, do not significantly influence the primary functional role of lubricants in providing necessary protection under pressure.