Methods include:

Filtration: Removing solid particles using fine filters.
Centrifugation: Separating contaminants based on density.
Vacuum Dehydration: Eliminating water and gases under reduced pressure.
Adsorption: Using materials like activated alumina to remove acids and oxidation products.
Electrostatic Separation: Removing sub-micron particles using electric fields.
These methods restore oil properties and extend service life.