There are many reasons to not let your Oil tank get to low besides running out on the coldest night of the year. A typical oil tank is about 275 gallons. The amount in the tank is measured by a dip stick with inch increments on it.
Most likely your furnace will stop running if you let the oil level get down to about 4 inches. This is because the feed line to the furnace picks up the oil about 3 to 4 inches above the bottom of the tank.
If you have an older tank, there is a good chance that impurities or sediment have settled out in the bottom. If water has leaked into the tank, it will lie on top of the oil and make the dip stick indicate a deeper fuel level than is actually there.
When you get down to the lower levels, there is a greater chance of sucking water and impurities into the feed line. This can clog the nozzle and prevent the burner from operating properly, making a service call very likely. You can save yourself the trouble by keeping the minimum fuel level above that 4-inch point.
Aside from buying fuel when prices are low, some of the best ways to save money on heating costs are to keep the furnace tuned, set back the thermostat at night (5 degrees or more) and air seal and insulate the house.
You can monitor your fuel use by checking the fuel depth frequently. But beware this can be deceptive because the bottoms of most oil tanks are rounded. This means the inch mark on the dip stick at 20 inches represents a lot more fuel than the inch mark at 5 inches. While it may take several days to drop from 20 to 19 inches, you could burn up the fuel from 5 to 4 inches quite rapidly in a day. You might find yourself without heat until you can get a delivery. You should not let the level drop below 8 inches so you won’t get caught short. If you have a regular oil provider, they may be able to supply you with a chart that estimates the amount of oil left in your tank according to the level on the dipstick.
Another option is get a full service oil contract where your oil provider will track your oil usage using a calculation of heating degree days. They will then schedule your oil delivery before your tank gets too low.