The SPCC regulation threshold of 1320 gallons of above ground storage refers to the oil storage “capacity”. It excludes containers of less than 55 gallons “capacity”. So if you have ten 55-gallons oil containers that are half empty, you have 550 gallons towards the 1320 threshold as far as SPCC regulations are concerned. Not 275 gallons.
You also need to count those oil-filled equipments that have more than 55-gallons capacities. Examples of these equipments include hydraulic systems, electrical motors and electric transformers of which you are the owner or operator.