Which property is most critical in evaluating a lubricant's ability to perform under high pressure?

Viscosity index is the most critical property in evaluating a lubricant's ability to perform under high pressure. This measure indicates how much a lubricant's viscosity changes with temperature. Under high-pressure conditions, a lubricant must maintain an adequate film thickness to prevent metal-to-metal contact, thereby minimizing wear and optimizing performance. A lubricant with a high viscosity index will demonstrate less change in viscosity with temperature fluctuations, ensuring that it remains effective across different operating conditions.

In contrast, thermal conductivity relates to the lubricant's ability to conduct heat, which is important but secondary to the viscosity concerns under high pressure. The flash point signifies the temperature at which a lubricant can ignite, which is crucial for safety but does not directly impact pressure performance. Lastly, the pour point identifies the lowest temperature at which a lubricant remains fluid; while important for cold start conditions, it does not address the lubricant's performance under high-pressure scenarios effectively. Understanding these distinctions helps in recognizing why viscosity index is paramount in high-pressure lubrication assessments.