The minimum viscosity for fuel transfer is determined by the maximum viscosity acceptable by the transfer pump. If the actual viscosity at transfer is lower than the pump design, the rate of transfer will be faster than the rated capacity of the pump. This arises because of the internal molecular friction within the fuel producing the frictional drag effect. There are two related measures of fuel viscosity which are known as dynamic and kinematic viscosity. In SI units the theoretical unit is the Poise.
Kinematic viscosity is the ratio of the absolute viscosity to the density.
In SI units, the theoretical unit is the Stokes. For either dynamic or kinematic viscosity to be meaningful a reference temperature must be quoted.