Dylan,
A Manual J is a calculation to determine what your heat losses (heating) and gains (a/c) are for your home. They measure floors, walls, doors, windows, they check out your insulation, type of windows, size, etc. and based on your location and annual temps, compute what you need for hvac needs. If done properly, they also take into account your return and supply ducts. Most folks here in my section of CT give you this "we do this all the time and we know our stuff" line when it comes to replacing your heating system or they just base it on what you currently have or base it on your square footage of the house.
Problem is, what you currently have is most likely oversized. Oversizing your heating and A/C system means the motor overcycles which burns the most electricity and the most oil. Each time an oil furnace starts up, it spits a whole lot of oil out, but then as it runs, there isn't that great amount of oil used. So you want something that runs longer, not starts and stops all the time. Cycling is also hard on the motor.
We had 6 folks out here to check out our home for a replacement furnace, three I ignored right off the bat, one guy was more interested in my dog. I compared the 3 'best' companies and started asking questions. I do my research upfront so knew things to ask, got a bit of attitude when I questioned 2 of the places, the 3rd sent out a tech for a manual J right away. I had already done some calculations on my own so knew what to expect. We put in a 90,000 btu furnace in the house with a medium size nozzle which provides 73,000 btus. One company wanted to put in a 125,000 btu furnace and one a 150,000 btu furnace; my old furnace was 125,000 btus. The bigger the furnace, the more oversized, the more cycling happens, the more oil is burned. The guy who installed my furnace, oil guy, said most companies WANT to install oversized furnaces because then they get to sell you the oil.
Not only am I saving in my oil usage with my properly saved furnace, but I spent an extra 300 to get a variable speed motor which has infinite number of speeds, not just three that are set upon installation. The motor runs based on what the hosue is calling for temps. Most times it will run using very little electricity at a low speed and will only ramp it up in the coldest of weather. With our rising electric rates here in CT, I figured the upgrade was worth the cost.