To remove grease stains on the collar of a leather jacket or coat, you can dab them with a cloth dampened with a mixture of 3 equal parts washing up liquid, ammonia and water. Rub with an old toothbrush. Rinse & dry.
You can also dab the stain with a clean cloth soaked in warm water and white vinegar. (One tablespoon of vinegar to a bowl of water).
You can also remove grease stains or sebum from the collar, by sprinkling them with talcum powder or diatomaceous earth (or diatomite). Wait several hours and then brush.
If the stains around the neck persist, wipe with a cloth soaked in ammonia diluted with water and then sprinkle with talcum powder or diatomaceous earth (or diatomite). Finally wax the leather with a clear wax.