A multigrade oil is an oil that meets the requirements of multiple grades — one “W” grade and one single grade, e.g., 5W-30 or 15W-40, etc. This grading system has the advantage over the ISO system in that it defines the viscosity performance of the oil at both the low-temperature range and the high-temperature range.

Why do we use multi grade oils?

A multigrade oil gives you the best of both worlds by maintaining its performance at high and low temperatures. All liquids thin out as heat increases but a multigrade keeps viscosity at optimum levels within a certain temperature range. (eg. 10W-40) Monograde oils do not have a low temperature requirement.

Does multi-viscosity oil get thicker when hot?

Viscosity of Motor Oil

Certain motor oils are designed to be thinner (less viscous) when cold and more viscous (thicker) when hot. For instance, multi-grade or multi-viscosity motor oils show a low viscosity at low temperatures and high viscosity at high temperatures.