The viscosity is a measure of the liquidity of the lubricating oil and is no indication for its quality. The higher the viscosity, the thicker the oil. The viscosity is applicable to all liquids.
According to the SAE standards, (= Society of Automotive Engineers) the viscosity of engine oil is described by a number between 0 and 60 before and after the letter 'W'. Gearbox oils can have numbers higher than 60, whereby the gearbox oil SAE 90W-90 would have the same viscosity as the engine oil SAE 30W-30.
The number before the letter 'W' (for 'winter') shows the viscosity at a temperature of approx.-20°C and is very important for cold starts and for cold running. The number after the letter 'W' shows the viscosity at about 100°C, i.e. when the engine is demanded highly. In the diagram shown above, two single-grade oils e.g., 10W-10 (green line) and 30W-30 (blue line) are depicted. The red line shows the behaviour of a multi-grade oil. (in this case 10W-30). In a cold condition it has a lower viscosity than the single-grade oil shown in blue and in a warm condition it is not as thin as the oil shown in green. The introduction of multi-grade oils has made oil changes in autumn and in spring unnecessary. One could also describe them as 'all-year' oils. 06/08