Is this the fluid in the powershift portion of the transmission, or does it have hydrostats? I think Cat is running hydros up to the D5, maybe even D6 sizes. If it is thick and white you have a water contamination issue that needs to be addressed first. Give a few more details, and I'll try and help more.
Oh wow, didn't realize how old that beauty was...but I still can't think of any fluid that would be thick and white without water contamination!