Depends on the oil you're using, for one thing. If multi-viscosity, I
wouldn't worry about anything down to 20 or so. Lycoming's web site
says 10F is OK, but my IO360 turns over pretty slowly below 20 - that
might be from a cold battery, though.

I think that a good approach if you live where it's really cold for an
extended period is to have both crankcase pad and electric cylinder
heater. Plug both into one of those cube taps that turns on when the
temp gets below 35F and just leave it on. Moisture does not form if
the engine is evenly heated, so wrapping the cowl is helpful, as is
wrapping the prop and spinner.