A single answer is not possible because there are many types of heat transfer fluids. Three types that I know of are glycols-water mixtures and glycols, hydrocarbon oils, silicon fluids.
Please give more deatils about the fluid used.
Another avenue for gettina answer is open to you if what was used was sold specifically as a heat transfer fluid. The manufacturers may have information on how to clean the system up. One example of this is the web site for Therminol® Information Bulletin No. 1 – Cleaning Organic Heat Transfer Fluid Systems, http://therminol.com/pages/tools/info1.asp.