By definition Viscosity is the measure of a liquid resistance to flow. For oils this is measured by ASTM D445 Kinematic Viscosity. The unit of measurement for viscosity is the Centistoke or cSt. Each grade of oil has it's own specific viscosity range based on its SAE grade. From baseline if an oil changes by greater than 10% it is no longer lubricating as intended. There are several factors that can change the viscosity of an oil. First Fuel Dilution and second Oxidation.
If your engine is operating excessively rich, the residual fuel that is not burned will make it into the oil system. This has the net effect of decreasing the viscosity of the fuel. Oxidation on the other hand is caused by thermally loading the oil for a prolonged period of time. As the index value for Oxidation increases you will see a corresponding increase in viscosity. (The typical limit for Oxidation is an index value of above 23)