Read this advice on how to improve the safety of your waste oil furnace. First, strain the oil in order to remove as much contaminant as possible. Use an electrical systems (essentially oil-based batteries, that attract nasty stuff to the anode or cathode) that clean up oil. You can work with a high-heat furnace, that delivers not only high heat, but also maximum airflow. Finally, you can modify a catalytic converter system to work with your system, so that any exhaust that comes from your system is cleaned. Just remember that catalytic converters run hot, so they should have stone beds of their own, which are located away from ignition sources.