A mixer with a helical ribbon impeller is an excellent choice for mixing a 100,000 cp fluid.  I have seen them used successfully for much higher viscosities.  They are an expensive choice for low viscosity fluids, but they still work quite well.  At the high viscosity, fluid density will have little if any effect on power input or mixing capability.  The high temperature may cause other mechanical difficulties, with seal materials and contacted drive components.
 
Helical ribbon impellers follow some general rules.  The impeller to tank diameter is usually between 90% and 95% of the tank diameter, partly dictated by the vessel size and practical clearance problems.  For a large diameter impeller, the vessel cross section must be very round and the mounting must be on center and aligned with the centerline of the vessel.  The pitch of the helix is usually equal to the diameter and the blade width is typically about 10% of the diameter.  A double flight helix is recommended to balance lateral loads when the fluid level does not reach the top of the helix.
 
The blade thickness will depend on the torque capabilities of the mixer drive.  All components of the mixer, blades, shaft, and mounting should be designed to be capable of handling the drive torque.  A variable speed drive is also recommended, because a helical ribbon impeller often works better at a lower speed.  If the speed is too high, the impeller may cut through the fluid, rather than circulating it for good mixing.

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