Are you thinking of buying, selling, or neither?
In the markets I work in New Jersey, I see a fair amount of underground storage tanks holding oil. These tend to make buyers nervous, even with a clean soil test. I have sold properties with UST that were leaking. I have dealt w/ high level contamination that required extensive EPA involvement and minor leaks that allowed the NFA (No Further Action) documentation to be completed by the company that removed and installed a new tank.
So, one concern is UST (Underground Storage Tanks) and another is efficiency of cost of Oil vs. Gas.
Many buyers want the UST to be replaced w/ an above ground tank. I have also seen several tanks in basements. If you have now, or are considering purchasing a property with a UST, by all means, do test it. In NJ, there is actually a grant program to assist homeowners w/ the cost of removal of the UST. If you are in NJ, contact me and I will provide you further details.
If you have an above ground tank, or one in the basement, oil can be an attractive alternative to gas.
Comparing costs and efficience is more than just a simple analogy of gas and oil, it is also a review of your current equipment and the efficiencies provided or lack thereof. i.e. A conversion may install new equipment that leads to less fuel consumption. But, you might also be able to achieve the same by simply installing new equipment without converting. Then, there are all the other factors such as insulation, window efficiency, etc.
I don't think most of Realtors are able to deliver a true evaluation of all of these issues. Call local suppliers for oil and ask them to provide you data. Here are a few links that might help you, also.
This first one is rather detailed, but if you skim the document there are probably useful pieces of info.
Here's a simpler document, although it comes from a trade group for oil heat.
Your best bet is to contact the local energy providers in your area. They probably have information on this for consumers.