Grease lubricants are categorized based on their consistency, which is measured by a standard called the National Lubricating Grease Institute (NLGI) grade. The NLGI grade ranges from 000 (very fluid) to 6 (very firm), with the most common grades being 1 and 2. The choice of grease consistency depends on the specific application requirements, including operating temperature, load, and speed. For instance, a lower NLGI grade is suitable for high-speed applications, while a higher grade is better for heavy-load, slow-moving machinery.