Multigrade oils typically begin as base oils, such as 10W. Then viscosity-index modifiers (polymers) are added in an effort to stabilize the viscosity. This allows a 10W40 oil to flow like a 10W at cold temperatures and a 40W at higher temperatures. In other words, multigrade oils are formulated to pass viscosity tests across a range of weights. For example, 10W30 meets the requirements for 10-weight at cold temperatures and 30-weight at high temps.

Oil thins when heated and thickens when cooled. Choosing the proper motor oil viscosity grade for the ambient temperature of your geographic location is therefore vitally important.

In a monograde oil the motor oil viscosity is defined at only one temperature, either high or low. A multigrade must keep a viscosity that will protect the engine effectively at both high and low temperatures.

This makes multigrades an easy and popular year-round choice for drivers who experience hot summers and harsh winters. Multigrades are easily recognized by the dual viscosity rating (i.e. 10W-30 where the 10W is the low temperature or winter designation and the 30 is the high temperature designation). It is the motor oil viscosity modifier additive that produces a thickening effect at high temperatures but is dormant at low temperatures.

Multi-weight oils (such as 10W-30) are a new invention made possible by adding polymers to oil. The polymers allow the oil to have different weights at different temperatures. The first number indicates the viscosity of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature.