Viscosity is a measure of a fluid's resistance to flow. For lubricating oil in general, viscosity is the most important physical property. It is viscosity, as well as the pressure and speed of movement, which determines the thickness of an oil film between two moving surfaces. This in turn determines the ability of the oil film to keep the two surfaces apart, the rate heat is generated by friction and the rate the oil flows between the surfaces and thus conveys the heat away.

The oil should have a viscosity at the operating temperature that is correct for maintaining a fluid film between the bearing surfaces, despite the pressure tending to squeeze it out. While a reasonable factor of safety is usually desirable, excessive viscosity should be avoided because this can create more drag and therefore unnecessary heat generation.

Viscosity is also useful for identification of grades of oil and for following the performance of oils in service. An increase in the oil’s viscosity during use usually indicates that the oil has deteriorated to some extent, a decrease normally indicates dilution with fuel. The permissible extent of viscosity increase before corrective measures are taken is largely a matter of experience and judgement of the operator

Monograde oils are designed to be used within a defined temperature range. The monograde oils are classified by the thickness (or viscosity) of the oil. The intention is to ensure that the oil will cycle as required under specific operating conditions. In cold climates a thinner grade, e.g. SAE 30, is more likely to be required whilst in warmer climates a thicker grade e.g. SAE 100, is more likely to be selected. They can also be characterized by the presence of an additive package ("AD" or ashless dispersant oils) or the absence of such performance additives ("S" or "straight" oils). For example, Mak Gold is a monograde oil.

Multigrade oils are designed to provide a more consistent viscosity across a wide range of temperatures. They are usually formulated around premium quality base stock (mineral and/or synthetic blend) enhanced with an additive package. Mak Multigrade, Mak Elite, Mak Auto XL, Mak 4T Nxt are all multigrade oils.

The Society of Automotive Engineers is an international non-profit educational and scientific organization dedicated to advancing mobility technology. They develop technical information on all forms of self-propelled vehicles including aircraft and disseminate this information through many channels, including oil specifications. In other words it tells you the "thickness" of the oil. The lower the number, the "thinner " the oil; thus SAE 30 is less viscous than SAE 40

This is the common terminology used to indicate a multi-grade oil. 'W' signifies the winter rating of the oil, showing that it will perform well in cold weather. The lower the number prefixing the 'W', the lower the temperature the oil can withstand. Thus 10W- indicates a lower viscosity at low temperature than 20W-. The second figure shows the viscosity at 100 C, which is close to the bulk oil temperature in most water-cooled engines.