To summarize:
You can break-in the engine in any temperature, but it is of critical importance to get the engine up to normal operating temperatures or hotter. This heating process is necessary to get the internals set and working together. It allows the metals to expand to their proper size and get their fit right.
If you do decide to break-in the engine during the extreme cold temperature months, make sure you use a little common sense and caution. One, pre-heat the engine (a hair dryer is a good tool for doing this). Getting the engine warm first will make starting it that much easier. Two, between runs do not let the engine cool down out of doors. Either get it back indoors or have the hairdryer back on it quick fast and in a hurry. Letting the metal components of your engine cool too rapidly will lead to metal fatigue and severe damages (ie warped conrods, cracked piston heads, cracked sleeves, etc.)
Finally, as a product of trying to break-in the engine during such cold temperatures, you will most likely need to run a few more tanks of fuel through it during break-in in order to get the engine ready for proper running.
I think NCNitro had the best idea...wait until the outside air temps are up then have fun with your toy.
