Mink oil, not even sure whether there's any actual mink oil in product labelled as such but use it to keep leather motorbike and walking boots supple and waterproof. I think it does a good job, don't use anything special to clean them, i.e. warm water only as required...
Tell me where you buy Grangers and I will give it a try -my dubarry boots need a new heel as current one has rotted so inner core structure showing but maybe a polish first.