For the majority of drivers the SAE viscosity class is the most important oil parameter. The SAE classification divides oils into one-season winter grade oils marked with 0-25 and letter W and summer grade oils marked with 20-60. Most frequently we use multi-season oils combining viscosity properties of winter and summer oils, i.e. at low temperature such oil will act as winter grade oil and at higher temperatures – as summer grade oil in its class.
In case of winter grade oils, the smaller value before letter W, the better the oil performance at low temperatures is. The higher the second value (following the letter W), the slower decrease of viscosity along with temperature growth . Oil marked as 0W-30 has low viscosity in the entire range of temperatures (thus it causes smaller resistance). Oil marked as 10W-60 does not flow down the cylinder walls even at high temperatures.
Currently all one-season oils (5W, 10W, 15W or 20, 30, 40, 50) in applications for vehicle engines have been replaced by multi-season oils (5W-30, 5W-40, 10W-40, 15W-40 etc.), adjusted to high needs of modern drivers. Multi-season oils are adjusted both to be used at high and low temperatures. The application of suitable oil does not only protect the engine, but also improves the comfort of driving and allows to reduce fuel consumption. In case of protection of fuel consumption such oils, as 0W20;5W20;0W30;5W30, are of special importance.
See also: Decoding oil viscosity (table)