Mixing Used Oil and Hazardous Waste
In addition to EPA's used oil management standards, your business may be required to comply with federal and state hazardous waste regulations if your used oil becomes contaminated from mixing it with hazardous waste. If used oil is mixed with hazardous waste, it probably will have to be managed as a hazardous waste.
Hazardous waste disposal is a lengthy, costly, and strict regulatory process. The only way to be sure your used oil does not become contaminated with hazardous waste is to store it separately from all solvents and chemicals and not to mix it with anything.
***
http://www.azdeq.gov/environ/waste/solid/6i.html#6j
EPA has looked at this issue closely, and has determined that only certain contaminants pose any significant threat to public health or the environment. As a result, EPA has established limits for the maximum concentrations for the contaminants of concern. These limits were set such that the emissions resulting from the burning of used oil containing these contaminants, at or below these limits, will pose no more threat to public health or the environment than the emissions resulting from the burning of a cleaner-burning fuel, such as virgin oil or diesel.
The maximum contaminant concentrations, including a limit on the minimum flashpoint (a safety consideration) that a used oil fuel may have, are referred to as the used oil specifications.
When Used Oil Is Regulated as a Hazardous Waste
Used oil will often contain halogenated compounds (halogens), many of which are listed as hazardous wastes. In many instances, the presence of these compounds in the used oil is the result of mixing with chlorinated solvents, such as brake fluid and other solvents. EPA has determined that used oil containing 1000 ppm total halogens is not a hazardous waste. Used oil containing 4000 ppm total halogens is a hazardous waste and is therefore subject to regulation as a hazardous waste under 40 CFR, Parts 260 through 266, 268, 270, and 124. For used oil containing total halogen concentrations between 1000 ppm and 4000 ppm, EPA has said it must be presumed that the used oil has been mixed with a list halogenated hazardous waste. But since not every halogen compound is a hazardous material, EPA has provided an option to the handler of the used oil to rebut the presumption of mixing, by demonstrating that the used oil has not been mixed with a listed halogenated hazardous waste. The demonstration may be made by testing the used oil with an analytical method from SWS-846, Edition III, such as EPA Method 8021, or by applying knowledge of the halogen content of the used oil in light of the materials and processes used.
***
http://www.ecarcenter.org/mn/mn-usedoil.htm
Used oil is not inherently hazardous, but if it contains certain additives, or if it has become contaminated with other solvents, it can fall under the hazardous waste rules.