The hatters here in the Lounge tend to not engage in aging/distressing the hats they make. You may get a response but don't count on it. I suspect it is something one of these types of hat makers discovered whilst experimenting and may not want to divulge a proprietary process. Most hatters learn the trade the old fashioned way......trial and error......lots of errors. .
I used to be a mechanic many many years ago. Thinking that wearing the hat while changing the oil a few times might do the trick.
I think you might be onto something, but it may not be uniformly distributed.
Actually it looks like that … it can easily be grease or similar ...
If that is the case then I have provided a way forward for you. You're welcome!
I ll give a try who knows.. hahaha
John Lofgren Monkey Boots Shinki Horsebuttt - $1,136 The classic monkey boot silhouette in an incredibly rich Shinki russet horse leather. 
Grant Stone Diesel Boot Dark Olive Chromexcel - $395 Goodyear welted, Horween Chromexcel, classic good looks. 
Schott 568 Vandals Jacket - $1,250 The classic Perfecto motorcycle jacket, in a very special limited-edition Schott double rider style.
If nothing else your car has fresh oil!
