I don't have the need to condition my vintage boards.
These vintage boards you see out there when made, I believe, where conditioned with a paste of wax and oil.
One I bought looked to be nearly new old stock. It is impervious to water.
My other one that has separated to a degree still retains its finish.
Another cross cut black walnut board I bought new I treated with mineral oil initially and haven't redone it at all.
I do dampen them with water when I work on them.
It seems, a lot of these vintage boards have shrinkage to them. It makes me wonder if they were fully dry to start.