[Kenth]

Because folks have done wrong or been ignorant for 50 years does not make it right today.

Viscosity

Viscosity is defined as the resistance of a fluid to flow.
The more resistance the liquid creates, the higher the viscosity.
The higher the viscosity, the higher the fuel consumption, engine
temperature, and load on the engine.
The most important aspect of an oil is its viscosity.
To create the correct hydrodynamic cushion for maximum protection for any given velocity, surface area and diameter/tolerance, you need a specific viscosity.
In the design of an engine this ideal viscosity is calculated and then recommended.
As mentioned above, an oil too thin will not provide enough hydrodynamic lubrication, and an oil too thick will not flow properly.
Eventually, as an engine wears, it may be necessary to compensate by slightly increasing this viscosity. “High mileage oils” do this by being in the upper portion of the range for a specific viscosity.
The following table shows the different SAE viscosities that meet engine design characteristics.