Used oil analysis is a scientific method for determining the condition of an oil and the condition of the system in which the oil was used.  It is very similar to  the analysis of blood in humans.  Certain conditions present in oil can be directly traced to abnormalities in the lube system, just as blood tests detect physical illness.  This science can be applied to any system that uses oil as a lubricant.  Problems with engine, hydraulic, gear case, and compressor systems can all be diagnosed through used oil analysis.