I bought an 'OEM Rocket' and installed on my oil tank several years ago.
http://www.sybasystems.com/OEM%20Rocket/oem_rocket.html
Inexpensive, accurate, and simple to install.
My 275 gallon oil tank is housed outside of the main house in an attached shed. I can check the oil level without leaving the comfort and warmth of my home, and the electronic technology is definitely more accurate that those mechanical oil gauges used on most tanks.
You can always measure the amount of oil in your tank with tape-measure and use this guide. Tape-measure must touch bottom of tank.
http://www.sippin.com/oil%20tank%20measure%20chart.htm
Also a handy link to check oil prices in central NH, but you can select for your area as well:
http://www.newenglandoil.com/newhampshire/zone3.asp?x=0
Interesting Fred Fuller isn't listed here?
Hope this helps everyone.....