When I was the chief pilot of a small Piper Navajo operation in northern Ontario Canada, we used the freezing temperature as the cut-off for pre-heat. Even the hangar was kept at 40*F overnight. We would push the planes out into -40*F and start them up right away. When out on a charter and waiting outside all day, it could easily be -25*F in the afternoon, so we would preheat the engines and use custom made engine blankets. It was too cold to use a camera, so I don't have any pictures to share
One thing we had to watch was high oil pressure after start. We'd have to idle the engines at 800rpm initially and slowly increase RPM to about 1200 as the oil warmed, thinned and the pressure dropped from the maximum limit. Leaning aggressively to prevent plug fowling.
Engines were Lycoming 6 cylinder angle valve and a big turbo (TIO-540) running multi-viscosity oil. They routinely made it to TBO.
FWIW, I'll probably preheat my plane at warmer temperatures. Not for any concerns about the engine, but to reduce warm-up time and save gas. Plugging in extension cords and setting up engine tents all takes considerable time that a commercial operation doesn't want to waste. On a private plane, I'd rather save that couple dollars of fuel not idling and warming up as long. I plug in my cars at home at 15*F for the same reason.
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