The federal government's Energy Star program is the most comprehensive rating system for furnaces. See their website for lists of the most efficient models, tips on sizing and installation, a savings calculator, and other resources.
When shopping for an efficient furnace, it is important to understand the annual fuel utilization efficiency (AFUE) measurement. AFUE is expressed as a percentage; the higher the better. Most furnaces on the market have AFUE ratings between 78 and 96 percent. Energy Star-qualified oil and gas furnaces have high AFUE ratings and are up to 15 percent more efficient than standard models.
Here are some other things to consider when selecting a new furnace:
In cold climates, a condensing model is the best choice. According to the American Council for an Energy-Efficient Economy (ACEEE), a furnace with an AFUE of more than 90 percent is "condensing." These high efficiency furnaces save some of the energy that standard furnaces waste by capturing and condensing water vapor. ACEEE recommends condensing furnaces as a first choice, except in warm climates. (For those in warm climates, ACEEE suggests retrofitting your system with a heat pump instead.)
Don't forget to consider the furnace's electrical efficiency. Furnaces can use a lot of electricity in addition to the fuel they burn. Most of this electricity goes to powering the fan motor. Instead of a standard ("PSC") motor, I recommend you choose a variable-speed motor-preferably two-stage variable speed. Two-stage variable speed motors are just built better. A two-stage variable speed furnace, no matter what brand, should save $200-300 per year on the electric bill alone. The variable speed blower uses only the equivalent of a light bulb's electricity while a standard furnace uses about 600 watts of electricity when it is running. Most homeowners will make their added costs back in about two to three years. A product directory of electrically efficient furnaces can be found on the Gas Appliance Manufacturers Association (GAMA) website.
Buy the best quality furnace your budget will allow. Top-of-the-line furnaces usually have a better quality igniter, blower motor, gas valve, inducer motor, and control board, plus insulated cabinets, resulting in a quieter furnace. Try to get a furnace with stainless steel primary and secondary heat exchangers. (Be careful-a lot of companies just have "stainless steel heat exchanger" written on their brochure and they don't specify that only the secondary is stainless steel.) Finally, try to get a furnace with a spark igniter over the hot surface igniter.
Pay attention to the quality of the installation, including proper sizing and ductwork. Remember that a furnace is only as good as its installer. A-top-of-the-line furnace installed improperly could cause you many problems down the road. Make sure your ductwork is sized properly, or you could burn through blower motors regularly. And make sure the furnace is properly sized for your home. Oversized furnaces are a common mistake that you can prevent by having your green HVAC contractor do an "ACCA Manual J" (referring to the professional guidelines of the Air Conditioning Contractors of America) or better heat loss analysis.
Oil versus gas? Finally, in response to the second part of your question, I recommend that you switch to natural gas if possible. Natural gas produces fewer harmful emissions, resulting in better air quality and less pollution.
For more information:
See GreenHomeGuide's Energy Efficiency Know-How section for more energy saving advice.
The ConsumerSearch website has furnace reviews and shopping tips.