The compatibility standard can be assessed using the ASTM D6185 (American Society for Testing and Materials) method, which determines the characteristics of the greases in the mixture: two greases are defined as incompatible if the chemical-physical characteristics of the mixture differ from the intermediate characteristics of the two starting greases. The principle of the ASTM D6185 test is to mix the two greases in different proportions under controlled and identical conditions and determine, after a short resting time at room temperature, any changes in structural stability from the original stability of the individual new greases. The compatibility of greases is also affected by operating conditions such as temperature, shear stress, the condition of the previously used and replaced grease, and the volume ratio of the greases in the mixture.

For this reason, a compatibility chart of different greases formulated with different soaps has been compiled, which provides a guideline on possible permissible combinations.

Compatible means that there is a LOW risk of structural instability; Incompatible means there is a high probability that structural instability will occur; 

But what happens in practice if I mix incompatible lubricating greases?

In reality, since these are semi-solid products, it is very difficult in normal applications to have conditions in which the greases mix homogeneously as in laboratory tests, and typically the contact between the products is rather limited, therefore reducing the real negative effects. Furthermore, lubricated systems tend to be designed to remove the grease present precisely to facilitate lubricant replacement.

When choosing a grease, the main recommendation is to follow the manufacturer's instructions. If forced to combine greases containing soaps classed as incompatible, the first step should be a very generous initial grease application in order to remove as much residue of the previous grease as possible, and if possible reduce the time to the next relubrication.