Keeping on topic, the most efficient fluid to use is
Novec from 3M. Do be aware that it is VERY expensive and you MUST ensure that it is used in a closed system as the fluid will evaporate.
Just do a search for '3M Novec fluid' and you will get several links for suppliers as well as links for other heat transfer fluids that can be used.
Another fluid to use would be from a company called
Engineered Fluids who are based in Texas. While their heat transfer fluids are mineral-based they have been tailored for use with electronic gear.