I have worked in an industrial field for quite a long time. Performing a viscosity test on used oil can tell you an awful lot about the condition of machinery like axles, Gearboxes, final drives, engines, and more.
These tests help us understand the level of thickness and flow in the oil that is sampled. The results give a good indication of whether or not the right grade of oil is being used for the right machine.
When oil acts as a lubricant, the viscosity is very important in making sure it can provide the right film thickness. If the viscosity is off, metal parts or components can rub against each other and cause wear and eventually lead to breaking down.