Degreasers are used for the removal of oil (usually from hair and skin) and oil based stains from leather. Generally, degreasing products are solvent-based materials that turn into an absorbent powder.
Apply degreaser to the leather. It dissolves the oils. A white powder forms on the surface. The dissolved oil absorbs into the powder. Oil and grease that build up over time cannot be removed in a few minutes. Your patience is necessary to complete the job. Also, stains left untreated for a long time may have caused additional challenges like loss of color.
Application of color products may be necessary. Use in a properly ventilated area. Read safety instructions.