Gregory- somehow I'd gotten you confused with the original poster from Long Island (as sometimes happens when you take over a thread. ;-) )
It looks like you used ~650 gallons of oil in all of 2010. With a zip code we can look up something more accurate better than the 25 year HDD averages from the NOAA, but for the time being we'll use that. A zip code would also be necessary determining the outside heating design temperature (which is not function of HDD) , but for the time being we'll use -10F (about the 97.5th percentile design temp for Millinocket.)
Assuming your boiler isn't a new-school lower-mass smart-controls system, with 3x oversizing and and an as-used AFUE of 75%, you're delivering:
(.75x 138,000= )103.5KBTU/gallon.
Over 8109HDD (base 65) that means the house needed (650 x 103.5K=) 67275 KBTU
or (67275K/8109=) 8.3KBTU per heating degree-day.
That's (8.3K/24 hours=) 0.346 KBTU per heating degree-hour, which is the same as 346 BTU/degree-hour.
With a design temp of -10F, you're looking at 65-(-10)= 75 heating degrees, so the heat load at design temp will be (at most) about
75 x 346= 25,950 (which would indicate a fairly small &/or a very tight house.) If rather than 3x oversized, yours is a perfectly right-sized 85% boiler you'd be looking at a -10F heat load of 29,500BTU/hr- call it 30K.
At -10F the Mitsubishi Hyper Heating (aka "Mr. Slim") units put out about 73% of their fully rated heating load, with a COP of ~1.5, so to do the whole thing with a mini-split would take a 3+ ton unit, but a 2-ton would handle your average load just fine. (Other vendors have somewhat different very-low temp performance, but both Daikin & Fujitsu make units that work fine at -10F as well.) The mean January temp in Millinocket is ~15F, a temp at which most mini-splits are running COPs of ~2.5, and electricity costs in most of Maine is about half of what it is on Long Island, so the dollar savings for YOU may be quite significant compared to those of a L.I. resident. During warmer months the average COP will rise, making it an even better deal, with an even larger share of the total load handled by the mini-split, but your seasonal average will still likely be about 2.5-ish since most of the heating energy will be used when it's under 20F, at COPs under 3.
Assuming you're paying 10cents/kwh and averaging a COP of 2.5, you're getting 8530BTU of space heating for every kwh, so using the 75% oil-boiler example that's a gallons-to-kwy equivalance to 103,500/8530= 12kwh, which costs you a $1.20, which is a third the price of your last 100 gallons, half your 2009 average. Have you seen $1.20 oil anytime in the past 20 years?