5.9 describe the trend in boiling point and viscosity of the main fractions

The lower the boiling point, the lower the viscosity, and vice versa (high boiling point = high viscosity)

NOTE: basically, the lower the viscosity, the thinner the substance runs, the higher the viscosity, the thicker something is (as liquid). For example, water has a super low viscosity, whilst syrup has a higher viscosity than water.