Despite all the advances of modern technology, the best solar fluid for use in a solar thermal installation is water. It retains and transfers heat better than any other liquid in the world. However, it has one major drawback: it freezes easily.
Consequently, the solar industry has had to come up with alternatives, usually based around mixing in an antifreeze (whether natural or man-made) to retain the excellent properties of simple water but still allow systems to operate at extreme temperatures. This has knock-on effects, such as increasing the difficulty of pumping the fluid mix.
Over the years, all sorts of chemicals have been tried in pressurized systems: propylene glycol, ethylene glycol, synthetic oil, silicone oil and alcohol. Unfortunately, ethylene glycol is toxic and deteriorates very quickly, damaging the system. The oils are not only poor heat conductors, but require specialized equipment and also eat into any rubber or plastic components they contact – definitely not practical!
The only acceptable option is a propylene glycol and water mix , and that's why it is today's industry standard. Propylene glycol is non-toxic and relatively stable at the high temperatures encountered in solar thermal systems. It's also biodegradable, which is a nice bonus in a renewable energy setup.
However, like all chemicals, propylene glycol comes in a wide range of types, and not all of them are suitable for your solar thermal needs. The most important feature is its resistance to heat: you need to choose a product that can stand up to constant operating temperatures of at least 350°F (177°C) without breaking down. Thankfully, most manufacturers produce products rated up to 370°F (188°C), so you're covered on that front.
Assuming the product will resist the heat, it's worth checking the extras: many manufacturers add other benefits to reduce corrosion, stabilize the mix and protect your solar thermal equipment.
Once you've picked a product, you'll need to look at percentage mixes and consider how it affects the rest of your installation.