Here in South Central Alberta, Canada, we fly our training
airplanes until the temp gets to -25 degrees C (-7 F). We use Aeroshell
15W50 exclusively. If the airplane is away on a cross country, and ends
up outside overnight, we'll probably preheat it if the temp was below
-10C (14F). Starting must be done carefully, as it's easy to kill the
battery, flood the engine, or get the initial revs too high. That oil
is a little stiff and doesn't pump readily when too cold, so keep the
RPM to 1000 or less until things warm up. We find it's best to have an
insulated cowl cover, and a car warmer stuck up into the cooling oulet;
it warms the engine, battery, everything.
40 and 50 F is NOT cold. Some of us, especially the younger
guys, don't even put a jacket on for that.

Dan