Oil breaks down or degrades with use. A lot of mechanics think they can measure the viscosity of an oil with their fingers to determine the quality or viscosity when it is used. Here we can see how the oil breaks down and can deceive us. There are 5 examples of how the Bolivian made Pennzoil looses its viscosity.
In the first example the oil has only been used for 160 hours in a relatively new engine. The viscosity is barely over the minimum for the 15W-40 grade.
Relatively new oil (only 160 hours)
Extending service life without having the necessary protection
Tricking the viscosity test
Poor quality oil doesn't protect in adverse conditions