The viscosity of an oil could be reported at any temperature, but to standardize things, most laboratories report either a low temp (100F or 40C) or a high temp viscosity (212F or 100C) and stick with either Fahrenheit or Celsius. To be more specific, it is the thickness of oil at a given temperature.
Commonly, though, we think of it as an oil’s thickness. Technically, viscosity is defined as resistance to flow.
We tend to choose an oil with a viscosity that we believe is correct for our particular engine, but would another viscosity improve or reduce the life of the engine? Can we pick and choose a viscosity outside the manufacturer’s recommendations? Most of us have only a vague understanding of viscosity.