What ghurd says is true.
Tranny Fluid will be easier to handle than water to do its properties of being able to stay within a set viscosity over a wide range of temps. Its pretty much self cleaning too, so you might think about getting some for free from a local flush/fill type shops. Try getting a small amount and soaking a couple cans with your epoxy on them that are stuck together just to make sure it won't turn the epoxy stuff into goo.
You could also use a more complex system that stores the warmed fluid in a highly insolated container that would then be used to transport that heat in the evening to a heat distribution item.
However, more complex can also mean possible less eff% due to losses.
The one main gain from using the oil is that unless your located in -20F weather for days on end, it's not likely to freeze, so a drain back system shouldn't be needed.
BUT there's the possible need for a pump to move the stored/heated oil to place needing heat etc...
Tranny Fluid can be a mess to work with too, it likes to find leaks and make messy puddles.
Hope this helps and doesn't stop you from trying.
Any use of even a little bit of the solar heating gains will go a long way to helping keeping the bills down along with that way cool feeling of saying "I did this"
Bruce S
PS, Don't do like I did, and get the idea that mineral oil and a small submersible water pump will work. Worked, but mineral oil gooped up the pump. wan't a good thing thing. Pump cost me $5, oil for 20fl.oz was $5. mess was all day. And the Tranny oil would destroy the cheap plastic they're made of.
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