Engine and gearbox oils are classified based on SAE classes. Industrial oils are classified using the ISO viscosity rating (VG).
SAE stands for the Society of Automotive Engineers, which is the association of automotive engineers in the USA.
The system uses different, numbered classes:
SAE viscosity classes
- 0 W to 60 for engine oils
- 70 W to 240 for gearbox oils
Distinguishing features within different classes are primarily the flow properties at low oil temperatures (cold start in winter = W) and at high oil temperatures. Engine oils are also characterised by the borderline pumping temperature and the HTHS standard values.
Distinguishing features are primarily the flow properties at low and high oil temperatures.
SAE classes of engine oils according to DIN 51511
The reference temperatures for the cold state are set at
-5°C to -30°C depending on the SAE class. The reference temperature for hot state is set at 100°C, even though engine oil can have a significantly higher temperature in the engine.
SAE classes with limit values specified for both low temperatures and at 100°C also contain the letter W (for winter) for the lower numeric value.
SAE classes of gearbox oils according to DIN 51512
The reference temperature for the cold state is between -12°C and -40°C (depending on the SAE class). At this temperature, dynamic viscosity cannot reach the value of 150,000 mPa s, which is practically the solidifying point.
Cold viscosity is measured in a Brookfield rotational viscometer. Similar to engine oils, SAE classes with their cold-state properties defined are marked with the letter W. Minimum viscosity at high temperatures is still measured at the limit value of 100°C.
The International Organization for Standardization - ISO - devised a viscosity grade (VG) comprising 18 viscosity classes in the range of 2 mm²/s up to 1500 mm²/s at 40°C. This covers the range from gas oil (diesel) up to high-viscosity cylinder oils.
As opposed to car lubricants, the viscosity intervals for these are narrower.
A single viscosity value is given at a reference temperature of 40°C with a tolerance range of ±10%. The viscosity index is not specified.
Thus, in the same ISO-VG, there can be differences in viscosity of different oils at high and low temperatures. In practice, however, such differences are less important and among other things they are possibly compensated by the tolerance of ±10%.