Waste Oil
From ZombieSquadWiki
Waste oil, while thought of seldom, is a viable alternative fuel source for producing heat.
Waste oil, while not useful for most purposes in its contaminated form, can be run through a simple filter and then burned to produce heat very efficiently. Forced air oil furnaces typically use heating oil (crude diesel fuel) and so are the easiest to accept alternative fuels such as waste oil and bio-diesel.

