There are many ways to improve the efficiency of an old oil boiler or furnace. First, find out if your system is oversized. A properly sized oil burner should run 45 to 50 minutes per hour when the temperature outside is the lowest it gets in your area. To downsize, ask your contractor to replace the existing oil burner nozzle with a smaller one. Reduce the nozzle only one size on a conventional cast-iron head burner. Do not reduce it below the minimum firing rate given on the manufacturer's rating plate.
Flame retention head burners do a much better job of mixing the air and fuel than old cast-iron head burners. They are now almost standard on new units and can also be added to most older ones. A high-static burner will give even better performance. Replacing the burner can increase the seasonal efficiency of an old oil-fired furnace by about 15%. The nozzle must be reduced at least one size and can be reduced even further while maintaining good efficiency. In any case, the flue temperature should not get lower than 400°F for an outside chimney. In the combustion chamber, a ceramic fiber lining can be installed over the present liner.
Always have an experienced contractor check the system's chimney or flue whenever a change is made.
Excerpted from No-Regrets Remodeling by Home Energy magazine
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I agree with the author, additionally however, I recommend adding some boiler/furncae control to maximize the use of BTU's that are not being used in the the boiler during periods when it is warmer outside, some examples include adding and outdoor reset, such as the Tekmar devices or a device that manages the heat load of the boiler/furnace itself, such as the Intellicon HW+ which is mfg by the same company as the Beckett Heat manager, but is more sophisticated, the cost can run as low as $200 for the DIY to as high as $1000 for an installed Tekmar. The Intellicon has many test cases that show that it will guaratee savings of at least 10%