You can lower the limit, but it will take longer for your hot water to recover during high demand. Not sure if there's any oil savings there, because the burner will run longer. I've tried a bunch of different combinations of high limit/low limit/differential settings without noticing any drastic effects on oil consumption.
The best thing to do, in my opinion, would be to install a hot water coil in your stove. Just don't buy one from Hilkoil (thermo-bilt) ....I think everyone knows that story by now.
But anyway ... if it's a cold-start, the coil will keep the boiler fairly warm ... and even up to temp when the stove is really crankin'. Mine have paid for themselves easily - even factoring in getting ripped off by Hilkoil.