Heat Transfer Oil is a coolant. Coolants are needed to flow through a device to stop it overheating. The oil does this by transferring the heat that the device is producing to other devices. The other devices either use or dissipate it.
The best type of coolant has thermal capacity. This is the ratio of the heat added to, or taken away from a device to the resulting change in temperature.  It should also have low viscosity, be inexpensive, non-toxic and chemically inert so that it doesn’t cause corrosion of the system.
The word coolant is usually used by the auto and HVAC industries. Industrial processing used ‘heat transfer fluid’.
- Halliday, David; Resnick, Robert (2013). Fundamentals of Physics. Wiley. p. 524. Retrieved 16 October 2014