A couple years ago I had a spot on carpet, on pre inspection the customer told me it had been there 5 years it was a foot lotion that had spilled, after trying various things in the arsenal I tried blood remover and it pulled it, remember it is called Blood and protien remover! Apperantly the lotion was made from animal fats.
She was and still is a happy customer! I don’t know the effects it would have on leather but it might be worth a shot if you run across it.