The point here is that RJ used the descriptor so often so that we would all visualize an oily scum on top of saidin. This is a literary devise not a scientific examination. He is illustrating that there was no way to reach saidin except for reaching through the taint. However, once a man had gotten past the taint saidin was no different that it had always been.

It also seems very unlikely that the taint and saidin could intermingle/mix given the opposing natures. Not to mention we are speaking of energies here, not liquids, despite all the imagery painting out Oceans or Rivers and such, saidin is an energy source not water.