could any tell me how we can filter engine oil and what is formula for that?
in middle east they heat used oil(second hand oil,from cars or ships) to break moleculs then they add acid which is not good for environment.
what we can add instead of acid?what is the way in europe?

Anahita,
Lubricating oils consists of two parts Base oil and additives. As it is being used, the additives degrade and the base get oxidised. In addition there will be contaminants.

To recycle used oil, base oil has to be recovered, remove impurities and then replanish the additives.

It is not a simple process of filteration. There are methods like vacuum distillation, thin film evaporation and then hydrofinishing for regeneration of base oil.

The base oil recovered can be blended to make different lubricants.

Heating would drive off any fuel/light ends. I believe they add sulfuric acid to cause flocculation - detergents/dispersants/VI improvers become insoluble removing soot/other particles as they drop out. There are solvents, short chain alcohols and ketones that will accomplish the same thing without the acid - filtration then evaporation of the diluted used oil leaves one with basestock, filtered out additives/soot and condensed solvents for reuse.

thanks a lot for your answers.
i want to do this experiment in a lab but without adding acid, could you please tell me what are exactly formula (solvents, short chain alcohols and ketones).
i know how to do it with acid but i don't know what should i add instead of acid?

Been awhile since I did the tests
Isobutyl acetate (IBA), methyl hexyl ketone (MHK) and methyl ethyl ketone (MEK) all worked to varying degrees. Believe I thinned the used oil 1:4 with solvent - passed through fine filter (0.2 micron) or centrifuged to get rid of soot. Beginning used oil dark, opague - resulting oil brown to amber. Evaporate off solvent or add some water to cause the solvent (water soluble) and oil to separate so that majority of oil can be poured off- believe oil lighter. IBA and MHK left alot of the additives intact (oil brown) - oil viscosity similar to new whereas MEK dropped most of the additives, oil amber with much lower viscosity. Believe isoamyl alcohol and pentanol/hexanol also worked