A quick google....
Here
4. COLD WEATHER STARTING. During extreme cold weather, it may be necessary to preheat the engine
and oil before starting. [no values given]
This
famous Lyco flyer says (page 15 of the PDF)
For most Lycoming models, preheat should be applied anytime temperatures are at 10˚ F or lower. The exception to this rule is the 76 series models that include the O-320-H, and the O/LO-360-E. These engines should be preheated when temperatures are below 20˚ F.
and uses the word "guideline". 10F is -12C and 20F is -6C.
Then you have piles of "urban knowledge" e.g.
here
Never attempt a start below freezing without pre-heating
Assuming they mean freezing point of
water, that is 0C.
If somebody finds a current Lyco document speaking of mandatory preheating below
+6C I will eat my EDM700
This is an interesting product.