We've started our airplanes at ambient temps as low as -25C (-13F)
without preheat, but they sure don't like it. We run Aeroshell 15W50,
and it gets thick at that temperature. The battery loses most of its
cranking power in those conditions.
Big dangers:
1. Cranking too long, overheating the starter, maybe burning it out.
Just replaced one like that. $460. Give up and go home.
2. Priming too much and too long before cranking. The fuel mists into
the intake manifold, but will coalesce onto the manifold walls and run
out into the airbox, creating a fire hazard. Priming too much will
result in a brief run and stop, frosting those cold sparkplugs and
shorting them. Whatever the order of checklist, the prime should
happen immediately before start, not three or four items earlier.
Remember that the vapor pressure of the fuel is really low when it's
cold and won't fire consistently at all. Adding more via the primer
doesn't help, it just increases the flooding problem.
3. Too low or too high revs after start. Too low and the oil won't get
thrown off the crank into the cylinders. Too high and the pump will
cavitate, being unable to suck that thick oil up from the case. Think
McDonalds milkshake. 500-700 RPM works for us. I wouldn't even think
of starting an engine at those temps if it had W100 in it.
4. Idle it for a long time to get the cylinders warmed up and the oil
viscosity down. Taking off on a cold engine is false economy and will
soon enough get you, either financially or fatally.

Dan