On an oil fired boiler (this is a boiler, not a furnace) operating at temperatures below 140F risk both copious flue condensation (enough to ruin a terra cotta lined masonry chimney) or even corrosive condensation on the heat exchanger plates.
Any oil boiler old enough to qualify for AARP membership will not be hitting anywhere near it's nameplate combustion efficiency, even when (as it probably has been) retrofitted with a better flame-retention burner. A new boiler with a new burner can hit 85% efficiency fairly readily, but for clay lined chimneys it's safer to de-tune it down to 80-82% (the flue condensation issue). If it's been running at 110F a lot or cold-starting frequently it might not be possible to even get the combustion efficiency that high. The steady state efficiency is most likely well below it's raw combustion efficiency, and the as-used AFUE even lower still:
Oversizing is a big efficiency problem with oil boilers. See System #2 in
Tables 2 & 3 in this document. Even when tuned to 84.6 combustion efficiency it's steady state efficiency is only 78.4 % due to big jacket losses (see Table 2), but when 3x oversized for the space heating load it's annual as-used AFUE is a paltry 65.1% (see Table 3.) That's probably your situation, maybe 80% combustion efficiency on a good day freshly tuned, 65% or lower as-used efficiency.
If you retrofitted it with a heat purging economizer control you might be able to beat 75% as-used AFUE, but probably not 80%. If you're not replacing the boiler the heat purge economizer would be a good investment (especially as a DIY project, if you have some electrician skills.) A pro might charge $500-1K to install one, but they're available for under $200. Almost all new oil boilers come fitted with this type of control in lieu of dumb aquastats, partly in response to that Brookhaven Nat'l Labs study. System #3 in that study was fitted with a heat purge controller, and even at 3x oversizing the as-used efficiency only dropped to 85.3%, only 1.3% below the 86.6% steady-state efficiency.
To figure out your oversizing factor, the gph of the nozzle that it's fitted with and the annual oil usage would be good enough to ball-park it, but most boilers installed in the 1960s were at
least 3x oversized for the heat load of the house. If you're on a regular fill-up service a "K-factor" stamped on the billing slips would also be a way to determine your actual load. If the last tech who tuned it up tagged it with the measured combustion efficiency that would be useful too. For more on measuring the heat load via fuel use,
see this bit o' bloggery.
