Hi greetings. Reversing grey hair to black is indeed among the tough results to show to a patient since this is a multifactorial problem. Since in most it's genetically mediated, it's tough to reverse or treat. Oils may not really help. However there are certain peptides and newer molecules application of which may be worth a trial. It's also important to rule out hormonal irregularities and nutritional deficiencies. Increase intake of oral anti oxidants. Meet a dermatologist near you for further advise.