If you have a thermometer put it in the basement and monitor that the temperature doesn’t fall below 45F. Most likely this will not happen as your boiler and the heat piping in the basement will maintain above 45. We generally are not concerned with inside oil tanks gelling because of this. If you had an outside oil tank I’d recommend having a 25-30% kerosene to 70-75% heating oil mixture in your tank to reduce the gelling at extreme low temperatures.

Set to your comfort level and leave that temperature on hold. Most people turn the temperature up during, if home, and down at night for sleeping. The down side to this during extreme weather is the boiler won’t circulate hot water while the inside temperature drops. This could lead to pipes freezing during that off cycle.

Second, if you keep your home at 55, I’d recommend going to 60 and keep it at that temperature until the cold snap is over, all day and night.

Yes, it’s to circulate the water in your system to avoid freezing.