Most common coolant is water.
Very pure deionized water, due to its relatively
low electrical conductivity. You can get the best from TVA. (Tennessee Valley Authority) I worked for them for years before I joined the Army and they have on-site DI water. We made it. Has to be used in boilers to make steam. You could use Heavy Water but I doubt you can get ahold of much if any. (Thats a joke by the way.) Water is usually used with additives like corrosion inhibitors and antifreezes. Good ones are ethylene glycol, diethylene glycol, or propylene glycol. Ethylene is common in the Auto industry and can be found in any auto parts store.
Second is Oils.
Oils can be used where water is unsuitable like around electrical devices. Oils have a higher boiling point than water so oil can be raised to higher temps. 100 degrees Celsius is the boiling point for water. You do not want to introduce high pressures within the container or loop of your system so this would be a reason to use oil as well. Mineral oils, Castor oil and Silicone oils can be both coolants and lubricants a in closed-loop systems for heating or cooling. I do not recommend Caster Oil as it is a vegetable oil and comes from the Caster Bean. This could possibly cause gunk in your loop. So don't use it. Mineral Oils are byproducts that are made from petroleum and depending on the viscosity can be great for a cooling system. You do not want a Non- Newtonian fluid as the viscosity can change or get thicker rather as they warm. A constant viscosity over a wide range of temp is the type you are looking for these are called Newtonian Fluids. As you turn you PC on and off or Overclock from normal you will see temp changes you need a constant viscosity to be effective. Mineral oil is going to be the best bet because it does not absorb water. (No white milkyness in your system). It doesn't conduct electricity and also makes a great personal lubricant...
Oil and water are both better than air cooling alone. Plus oil is good for your pump, safe for you and your PC. You can even mix in some UV dye. So to be safe and sure for normal operation or overclocking use an oil!
Remember to alway use a vented system when using water and be safe!
For super overclocking usea liquid gas! VERY DANGEROUS! You could loose a hand or worse, but you can get world record numbers! Is it worth the risk? Well that another post. lol
Good place to get you oil! STE Oil Company**|**White Mineral Oil & Spraying Equipment
they have an 800 number and a shopping cart! lol
P.S. Remember this oil will never match water in thermal conductivity and specific heat capacity, but it is much safer and is still far better than air. Although, this could be corrected with a larger Rad., Res. and head pressure on your pump than you would use with water. Giving the oil more time to shed heat. There is alot you could do to improve your oil cooling. I would like to hear your ideas.