centrifugal vs positive displacement pump

The biggest issue is with pumping efficiency; with higher viscosities there is a lot more impeller friction and blade losses. When looking at pumping efficiency only, the economical viscosity limit for centrifugal pumps is about 150 cSt - while a general limit of 1000 cSt is considered as a maximum viscosity for centrifugal pumps. They can be used for even higher, but, the elevated viscosity decreases head and efficiency too much, and the power consumption of the pump becomes too much.

The second biggest issue is with air entrainment - highly viscous liquids usually have significant amounts of air in them that can lead to problems with centrifugal impellers.