Hello
when I speak of "clean" used oil I am speaking of clear oil.
for me it is hydaulic oil it is very little pollutant.
It is true that it is necessary to avoid the black oils of crankcase very loaded it by polluting.
I quote the ademe:
* Black oils: these include used motor oils and certain industrial oils used, for example, for hardening metals or as heat transfer fluids. These oils are severely degraded and contaminated.
* The clear oils come from the use of the lubricant categories mentioned in the table above. They are considered to be low in impurities and polluting elements and therefore easy to recover in material form.
The used engine oils analyzed at the outlet of the crankcases contain a certain number of pollutants resulting from the degradation of the original constituents of lubricants but also in contact of the oils with the fuel and the exhaust gases:
* soot, resins,
* heavy metals,
* organic acids resulting from the partial oxidation of the oil,
* chlorine from certain lubricating additives,
* aromatic compounds including polycyclic aromatic hydrocarbons (PAHs),
* phenols, phthalates.
Used black industrial oils are also very degraded and their contamination is similar to that of engine oils. Quench oils, for example, become loaded with tars and resins during their use due to the significant oxidation of the lubricant.
Finally, used clear industrial oils are not very contaminated. In fact, water and particles are generally responsible for the wear of the lubricant.
source :
http://www2.ademe.fr/servlet/KBaseShow?catid=14777