Fuel oil can be categories into 6 classes from Fuel Oil No.1 to Fuel Oil No. 6 which is based on its boiling temperature, composition, and application of uses. Boiling point ranges from 175 to 600 degree Celsius and it goes on increasing as we move towards the higher class. Viscous property is also increasing with the class and in terms of price it will drop down with increasing the fuel class number.