Actual efficiency of an old oil furnace?
There is no way to determine with any certainty what the AFUE was for the boiler in the past. A service technician can check the flue gasses with a flue gas analyzer and determine what the AFUE currently is.

A boiler can keep operating near its original efficiency as long as it is properly maintained. On an annual basis, at minimum the following things should be done: replace the oil filter, clean out the boiler and chimney, and replace the nozzle.

Basically everything on a boiler is replaceable. If you have the knowledge, you could theoretically keep a boiler going indefinitely. You can even replace the lining on the firebox if it wears out. The kits to do this are relatively inexpensive and can increase the life of the burner for many years.

If the boiler is still running well, there most likely is not a need to go out and replace it right away. Even the new high efficiency oil burners are only rated between 86 and 87 AFUE. It would take many years to make up the cost difference.