Mine have seen -21F without issues!
I used the 5119K841 on my 86 ...

I didn't touch them so I don't know at what point they become brittle, I suspect -15F is where they stiffen and become inflexible.

Considering they have no movement I suspect they'll be fine much colder, but I have no facts to back that assumption up with.

They also have two different durometer hardnesses. 60A=Soft and 75A=Hard.

The numbers I gave above were for the Hard (75A).
I might try the Soft next time.

I just installed a Pensacola kit with Viton o-rings on the 7.3 thats going in my 86 so I am not going to use Viton hose on it at this point in time ... I don't care about this motor, it's temp till I fix my 6.9 RIGHT! (Shhh, Don't tell my ol'lady but I might just blow it up so I'm forced into fixing my 6.9 sooner ... )

If you look at McMaster-Carr Viton® Fluoroelastomer they state:
<table><tr><td>Rubber</td><td>Viton® Fluoroelastomer</td></tr><tr><td>Material</td><td>Viton® Fluoroelastomer</td></tr><tr><td>Shape</td><td>Single Line</td></tr><tr><td>Reinforcement</td><td>Unreinforced</td></tr><tr><td>Maximum Pressure Range, psi</td><td>Not Rated</td></tr><tr><td>Low Temperature Range</td><td>-99° to -1° F</td></tr><tr><td>High Temperature Range</td><td>+301° to +500° F</td></tr><tr><td>Operating Temperature Range</td><td>-15° to +400° F</td></tr><tr><td>Bend Radius</td><td>Not Rated</td></tr><tr><td>For Use With</td><td>Ethylene Glycol and Water</td></tr><tr><td>Fittings Used</td><td>Barbed</td></tr></table>

-Enjoy
fh : )_~