Fluid Trainer in an Unheated Garage

Hmmm never thought of that one since I use mine indoors. I'm not sure what the fluid is inside of fluid trainers but it may be worthwhile to contact the manufacturer unless others have done this.

The only reference I could find in the Kinetic manual was "do not store in extreme temperature conditions" or something like that. No specific temps given.

If you're using virtual power, it will read low (you're really working harder for the speed, and therefore power, calculated) if the fluid is really cold. I think though that Kurt says it's pretty stable over temperature. They all warm up though with use.

The big question to me though is the seals; if they are frozen, that may make them more likely to leak. I think the Kurt units are OK, as they are pretty well sealed up, since they use magnets to connect the input shaft to the resistance unit (where the fluid is)

I think other brands though rely on mechanical seals, which may leak under low temperature conditions.

I have stored trainers in my garage here and there. Never had a problem but usually brought them in to be used. I have also left one in my car many times in very very cold temps (-30) and it was fine

I've had the same one in my garage (which is my workout space) for ~4 years now and it seems fine. I use it very regularly year round and I've never noticed any temp related issues.

Unless you are in an extreme cold area you should be fine.
The silicon fluid should be fine at low temps, without knowing the exact fluid they use I can't tell you how low.
The seals are generally good down to -30 to -40°F where they will take a temporary set until they warm up again.

Not a problem. The viscoelastic oil in the fluid trainer doesn't really freeze, so you won't have to worry about leaking. But it does increase the resistance (the oil becomes thicker in cold), so give the trainer a good 10 min of a easy warm up before you start your actual training set.