What do the numbers and letters mean, and how do I use them to find the right viscosity grade for my engine?

Viscosity

Viscosity is the single most important property of an engine oil and basically determines whether the oil will be suitable for use. It is a measure of the oil's resistance to flow. The more resistant or 'thick' the oil, the higher its viscosity.

A system for rating the viscosity of oils was established in 1911 by the SAE (Society of Automotive Engineers). An oil's viscosity rating is often referred to as its 'grade' (or in times past, its 'weight'). An oil that flows more quickly has a lower viscosity and, consequently, is given a lower rating. For example, an SAE 30 grade flows more quickly than an SAE 40 grade.

Multigrade oils are formulated to be thin enough to flow rapidly to areas requiring lubrication when the engine is cold, yet thick enough to protect the engine at higher operating temperatures and loads. Multigrade oils can be easily identified because they have a ‘W’ (for ‘Winter’) between two numbers. The number before the ‘W’ is the oil’s viscosity when cold. The number after the ‘W’ is the oil’s viscosity at an operating temperature of 212°F (100°C). For example, a multigrade SAE 10W-30 oil will flow more rapidly than a single grade SAE 30 oil in cold conditions, yet both oils will flow the same at higher operating temperatures.

Following the engine manufacturer's suggestions for viscosity grade is a wise choice when selecting an oil for normal driving conditions. When driving in cold climates, a multigrade oil such as a 0W-, 5W-, or 10W- with good start-up and flow behavior is important. Older engines that have experienced wear may be better protected with a higher operating viscosity grade oil. Engines that operate in a warm climate may only require a higher viscosity, single grade engine oil such as SAE 30.