Dilatants - Their viscosity increases with agitation. This is how cream becomes butter. Some clay slurries have the same problem. The resulting high viscosities will restrict the free movement of the seal. When dealing with dilatants it is important that you do not continually rotate the fluid in the stuffing box area.
Thixotropic fluids lower their viscosity with agitation.
They seldom present a problem for mechanical seals except for an increase in seal face wear. Plastic fluids change their viscosity suddenly. Catsup is a good example of this type of fluid. Newtonian fluids do not change viscosity with agitation. They present no problem for mechanical seals.