Oil usage is a function of the weather and your thermostat setting. A lower water temperature will make the pump run longer and may not heat the house if the weather is very cold. But the burner will only turn on occasionally when the water gets to the lower limit. You could set the water so low that the burner never came on and would freeze in the process. A higher setting will allow the pump to run for a shorter time and should satisfy the thermostat setting. If the burner should run for too long a period then it is because the thermostat is set too high or the weather is too cold. But in the long run, if you have the thermostat set at a comfortable level that the boiler water temperature is capable of satisfying, the oil usage will be a constant at most reasonable water temperatures.