How to calc. visc. of crude oil-water mix?

The issue of rheology of water-in-oil emulsions has been treated in specialized books. From an old one on viscosities of dilute liquid-liquid emulsions (as colloidal systems):

visc of mix= v_s(1+2.5*f_v)

v_s is the viscosity of the larger continuous phase (crude oil)
f_v is the volume fraction of the smaller dispersed phase (brine).

It remains to be seen whether this is applicable to the case in hand.

This is really a complex issue. To start with, you can use the following rule of thumb:
- Crude water mixture with water cut>80%: assume mix viscosity = 0.5 oil viscosity - water viscosity
- Crude water mixture with water cut 40-60%: asuume mix viscosity= oil visosity
- Crude water mixture with water cut<40% and >30%: assume mix visosity = 1.8-2 times oil viscosity
- for lesser water cuts: assume same as oil viscosity.

The point to be kept in mind is the viscosity inversion point varies for different crude oils.

Oil and water are both generally treated as Newtoniqain fluids but a mixture or emulsion of water and oil is non-newtonian and the viscosity of the resultant emulsion can be significantly higher than that of either of the components.
Fuel water emulsions are increasingly popular for large diesel engines (for pollution control) with emulsions of as much as 40% water. You may find companies such as SIT have some useful data on viscosity they can share with you or maybe some of the large engine manufacturers the same. You might try the MAN B&W engine research centre in Copenhagen as one source.
These links may help: