[M.D. Holloway]

Yup, stands to reason...Perhaps the single most important characteristic of a lubricating oil is the viscosity.

The viscosity of a fluid is a measure of its resistance, it corresponds to the informal notion of "thickness". For example, honey has a higher viscosity than water.

Viscosity is due to friction between molecules of the fluid that are moving at different velocities. When fluid is forced through a tube, the fluid generally moves faster near the center and very slowly near the walls. A liquid's viscosity depends on the size and shape of its molecules and the attractions between the particles.

The thicker the oil the greater the restriction and the increase in shear which translates to increased temperature...