Best way to very sympathetically "fill in" or "touch up" patina on old black Iron Tsuba

[BKB5]

I'm a jewelry maker from way back and still get BLOWN AWAY at what artistry the Japanese can accomplish in small metal work. 

Any suggestions from Tsuba restoration folk on "really gently and sympathetically adding back the thick, chocolate, sheen, patina in spots on some older Iron tsuba that were "cleaned" or treated with some kind of chemical (sigh).

I know from my years that there are as many ways to give as many patina's to Iron as there are stars. One way is hot oil - "yep oil, burnt, nasty motor oil" can be used with iron to "blend" - or - actually soak a totally almost ruined piece. light waxes and light oils (skin oil is very exacting, slow, can be used with color), and bluing compounds mixed or greatly watered down (or both).

 

There have been some "big, old, fantastic tsuba" for sale over the years with patina damage - I'd like to at least lightly experiment on one (I am careful, slow, authentic, and would be doing this for mine own eyes).

Thanks!!!@!