Lubricating greases are used to prevent metal-on-metal contact in moving parts, such as bearings.  Generally, a grease is made of 70-95% oil, 3-30% thickener and 0-10% additives.  The oil provides lubrication by forming a thin hydrodynamic film between the moving parts to prevent the moving parts from directly contacting one another.  The thickener includes solid particles or fibers which hold the oil.  Additives enhance or suppress certain grease qualities. 
Oil separation or bleed out occurs when the oil is no longer held by the thickener, which reduces the lubrication qualities of the grease.  Previous methods for determining oil separation from grease included the Standard Test Method for Oil Separation from Lubricating Grease by Centrifuging (Koppers Method—ASTM D4425) and the Standard Test Method for Oil Separation from Lubricating Grease (Conical Sieve Method—ASTM D6184) which subject a grease sample to centrifugal forces and higher temperatures, respectively, to test the oil separation from the grease sample. 
Different types of greases can be combined to create a grease combination.  However, not all greases are compatible with one another and incompatible grease combinations can lead to ineffective lubrication and damage of the lubricated components.  Moreover, incompatible grease types may clog up the pumps and valves of a lubrication system, damaging the system and preventing the system from distributing the grease.