I've been experimenting on a small flow simulation for the oil spills... here is a screenshot and small video of the thing in action:

http://photos-eric.chez-alice.fr/oil.../spilltest.avi
For testing purposes, the heightfield is a slightly inclined sinusoidal field, ultimately, it'll be the regular terrain heightfield.
The simulation is reasonnably fast for limited oil spills (in the demo, worst case is at 3 ms of CPU time per step, plus 1.5 ms to update the textures), but it's not negligible... so high-quality spills will most likely be limited to dual-core CPUs.