Cooling fluid

One possibility would be some of the Dowtherm or Sylterm fluids. See:

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Whats wrong with using a good, old fashioned, mixture of propylene glycol (or ethylene glycol) and water?

Pure propylene glycol freezes at -59C and boils at +188C. A 60%/40% mix of p. glycol and water will have a freezing point of about -48C and a boiling point of around +127C. Conventional antifreeze mixtures are cheap, readily available, and can be purchased pre-mixed with corrosion inhibitors.

See:

Paul: There is nothing wrong with glycol antifreeze and water, provided that you take care of it and periodically replace it to make sure that corrosion is being properly inhibited. If not, then woe may follow. I recollect an early aluminum auto engine failure due to corrosion.

Pittsburgh Pete