Did lead counter flashing on a very irregular chimney and to keep the white oxide from staining, it is common to use patination oil - at least in the UK. I guess it’s not used in the US - all sources of it on eBay are from the UK.
So there must be a substitute product. If I had to guess, I’d say linseed oil.
NOTE: I’d like to make it white. Paint will get pushed off by the oxide formation. However linseed oil can be pigmented so maybe …