NLGI grease grades range from 000 to 6 and are defined in ASTM D217, cone penetration test (full scale). The grades are determined by dropping a cone in a cup filled with worked grease and measuring the distance that the cone falls into the cup in a defined period of time (five seconds). The distance is reported in tenths of a milimeter as the penetration. The only characteristic defined by the NLGI grade is the consistency of the grease. Other very important characteristics of grease like chemistry and performance are not defined by this parameter. Softer, or lighter, greases have higher penetration numbers than stiffer, or heavier, NLGI grease grades.
When mixing two different greases, it is important to consider grease compatibility. Compatibility of different greases mixed together is complicated by the chemistry of the thickener, base oil, and additives. Some thickeners can become unstable when mixed with incompatible chemistries, resulting in the thickener becoming less efficient. When thickener is made less efficient, the mixture is softer and the grease may not stay where it is intended, separate oil at a much higher rate, or some other thing may happen that causes an application to become lubricant starved. Testing can be done to determine if two different greases that may be mixed together may be incompatible. Because this testing does not capture all of the possible failure modes, the safest course of action when changing grease is to remove as much of the grease as possible before making a change.