Applications of pumps and types of fluids.

Most of you must be aware that while water, is moved about by centrifugal pumps, (dynamic type), oil is moved about by gear, gerotor, or anyother positive displacement type of pump. why is this ?

Has it got anything to do with the viscosity of the fluids, or the area of usage ?

Also, the centrifugal pump has three types of impellers, and they are , with two crowns, with only one crown, and with no crowns at all.

the last one is used for sewage water, or water with lots of impurities, while the first one is used for pumping only pure water. now what has the crowns got to do with the purity of water.