Why don't you try this one:

 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

v1, v2 = velocities of particle 1 and 2
p1, p2 = locations of particle 1 and 2
dt = the time step

Dist = vlength(p1-p2);
v1 = v1 + Fluidity * (v2-v1) * exp(-Dist/Range) * dt;

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

this will cause all the particles to approach the same velocity 
exponentially within approximately 1/Fluidity time steps.  You could 
then use Range to control the effect of distance or the "skin depth" of 
the fluid.

Implementing viscosity like this might allow you to use a very gentle 
force to keep the particles together, like

v1 = v1 + ( Fluidity1 * (v2-v1) + Fluidity2 * (p2-p1)*(Dist-Size)  )* 
exp(-Dist/Range) * dt

In this case, Size is the average distance the particles will try to 
keep from their nearest neighbor.

Let me know if either of these ideas work.

- Ben

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