Unless you can do your own heat loss, forget those that would just replace what's there.
An outdoor reset controller does two things: lowers energy use, and makes the house more comfortable. A boiler that is 'right' sized and one where you can adjust the supply temp to the needs, means that it can run longer. Longer runs mean fewer cycles, fewer cycles means the whole thing lasts longer and provides more even heat, and, wastes less fuel at the end of the burn. Ideally, the boiler would run full-time, but not sure if any of the oil burners are modulating like what's available with gas, but a smart controller would shut the burner down so that the house just reaches temp as the water temp reaches the low point where it's useful for radiation.