Question

should a good lubricant be highly viscous or should have low viscosity and why?

Answer 1

Since viscosity is the measure of the fluid's resistance to flow, so extra power or effort will be required to overcome that extra resistance caused by very high viscous lubricant. Now, suppose you have a high speed bearing, you can not use the same lubricant that you used for a low speed bearing.

Answer 2

It decreases (thins) with increasing temperature and increases (or thickens) with decreased temperature. The benefits of oils with a higher viscosity is that a general increase in viscosity at higher temperatures results in lower oil consumption and less wear, which is profitable for the environment.