They are classified and named according to their viscosity, IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively. In the MARPOL Marine Convention of 1973, heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C.
They are classified and named according to their viscosity, IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively. In the MARPOL Marine Convention of 1973, heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C.