Engineers determine the appropriate viscosity of lubricants based on the workload and speed of the vehicle. Thin or less viscous lubricants generally have a low resistance, save fuel and offer high engine power while highly viscous lubricants offer good durability at contact points between metal surfaces.
One difficult to control variable is that the viscosity of a lubricant will change according to temperature. Lubricants become clearer or thinner at high temperatures and denser or thicker at lower temperatures. At low temperatures, lubricants must be free-flowing (not too dense or thick) whereas at high temperatures, lubricants need to maintain their viscosity to prevent friction between metal parts. In order to resolve this problem, engineers have developed multigrade lubricants.

Viscosity is a measure of a liquid's resistance to flow. A low viscosity liquid flows freely and its texture is 'thin'. Water is an example of a liquid with quite a low viscosity. Highly viscous liquids are 'thick' in texture. Syrup is a good example of a liquid with quite a high viscosity.

Lubricants are classified using a two letter code. Petrol engine lubricants start with an 'S' (formerly 'Spark Ignition', but now 'S', which stands for 'Service'). Diesel engine lubricants start with a 'C' (formerly 'Compression Ignition', but now 'C', which stands for 'Commercial').

The second letter indicates lubricant quality, which is improved over time. That is, when a new quality level is established in an industrial group, the next letter will be selected (SJ will replace SH). The letters 'I' and 'K' are omitted to avoid confusion with other widely used acronyms.

Viscosity index (VI) is a measure of the relative change in viscosity of a lubricant over a range of temperatures. A higher viscosity index indicates a smaller change in viscosity within a certain temperature range. VI does not correlate with actual viscosity or viscosity according to SAE standards; it is used to measure variability in viscosity.

VI is usually only used as a preliminary reference. Actual performance is based on compression test results at low temperatures and lubricant wear tests over a high temperature range. These test results are a better indication of actual performance inside an engine.

Multigrade lubricants (OW-40, 10W-30 etc.) generally have a high viscosity index while monograde lubricants (SAE 30, 40 etc.) have a lower viscosity index.