To bring a certain order to the highly different viscosities of conventional mineral oils they are grouped into classes with a defined bandwidth (according to DIN 51 519 = ±10 %). Equally, the viscosities named in this are related to a uniform temperature for reasons of comparison. This used to be +50°C according to DIN 51 502 and has been +40°C since 1977 according to ISO 3448 and DIN 51 519.
Thus, the old viscosity class 25 had a bandwidth of 32 to 40 mm2/s at 40°C, which becomes viscosity class 32 according to the ISO, which ranges from 28.8 to 35.2 mm2/s at 40°C (Figure V 30).
Figure V 30: Old (right) and new (left) viscosity classes