Viscosity is the opposite of fluidity: it is the resistance of a liquid to its own flowt. Elle It is read in comparison to new oil. A good lubricant should have multi-grade viscosity, c
meaning that it remains pumpable when cold (so fairly fluid) yet still resists very high temperatures without breaking up. The grade of an oil is always mentioned on the can (e.g. 5W40, 5 being the viscosity number when cold and 40 when hot). The viscosity number will change with use and this test serves to check how much it has changed. It is run at a temperature of 100°C and measures the time taken for the lubricant to flow into a graduated tube compared to the time a new oil of the same grade takes.

This test reveals two observable facts:
- an increase in viscosity implies the running temperature is too high or the presence of carbon particles due to poor combustion;
- a decrease in viscosity implies lubricant shearing or a large amount of unburnt fuel.