In the early 1960s, an improved asphalt grading system was developed that incorporated a rational scientific viscosity test.
The testing bath temperature must be 60° ± 0,3 °C,(average road surface temperature during sunshine) and 135 °C (average temperature of bitumen when mixing asphalt).
In hot climates, bitumen with low penetration and high viscosity is used (eg VG40), so it will not flow (deform) with the high ambient temperature., and bitumen with high penetration and low viscosity is used in cooler regions. so it will not “crack” under freezing temperatures (eg VG10) .

TestViscosity
VG-10VG-20VG-30VG-40 
Dynamic viscosity @60°c (poise)1000±2002000±4003000±6004000±800 
Kinematic Viscosity @135°c(Min, Cst )250300350400 
Penetration ,25°c,100g 5s,min80604535 
Flash Point °C220220220220 
Softening point °C40454750 
Solubility in trichloroethylene (min %)99.099.099.099.0 
Test on residue from thin film oven test____ 
Ductility ,25°c,5cm/min/cm75504025 
Viscosity ratio @60°C Max4.04.04.04.0