When its my own dime I plug in when its +20*F sometimes, depends on if its windy since that wind will be pulling heat away.
Right, and quite a few "keyboard mechanics" have argued wind chill only applies to living flesh. Park your truck on a 10° day in the sun with no wind and the cab will be way warmer than the same temp on a windy day. Even with a dark color the cold wind sucks the heat right out of the sheet metal.
Heavy truck and diesel mechanic (thankfully retired after 30+ yrs)
'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.