If the fluid is that sludgy, and you do not get a sufficient quantity out, you should
a) do repeated flushes with fresh fluid till it all comes out (maybe 2-3 times), or
c) repeat the drain & fill at shorter intervals till you start getting full volume out, or
b) run some solvent like kerosene (not too much of course; saw this done over on the Edge forum)
What kind of pump were you using, wasinger?