Hello Gabriel,
The temperature rise of the fluid flowing past the motor (regardless of viscosity) is a funcion of the head generated by the pump, pump efficiency, motor efficiency and specific heat capacity of the fluid. The Formula is:
ΔT = [H(1-Em)] / [778.c.Em.Ep]
Where
ΔT = Temerature rise of the fluid (oF)
H = Head generated by the pump (Ft)
Em = Motor efficiency
Ep = Pump efficiency
c = specific heat capacity of the fluid (BTU/lb/oF)
When you consider an ESP pumping high viscosity fluid, the pump efficiency plummets (10-20% depending on viscosity). Decreasing the pump efficiency decreases the denominator in the equation above, hence increasing the temperature rise of the fluid (compared to pumping water or light oil).
For a further explanation of the derivation of the equation above, see textbook "Electrical Submersible Pumps Manual - Design, Operations and Maintenance" by Gabor Takacs (this is a great text book for ESP design)
With this high temperature rise, you will need to make sure you can keep the core of your motor cool.
I hope this helps
James Rhys-Davies
Design Engineer
Hi-vis Pumps
UK