Stanley Rule and Level folding rule

[Northwoods]

Have a nice solid 4-section rule that has brass edges when it folds up.
But it is badly stained.  Oil?
What is the best way to clean it up?  Don't want to do more harm than good.

[Jimmy Joiner]

Hello Northwoods, I’ve used Barkeepers friend on my folding rules with reasonable results, just gently for the results I want, which is top layer of grime off whilst retaining some patina. I’m not sure with oil though. I’m impressed with your American BKFriend, it’s very fine grit.
Jimmy

[Bill Houghton]

If it's oil, try slathering mineral spirits all over it and then burying it in cheap (non-clumping) kitty litter, repeating as needed until you've got as much as you can from it.

If that's a boxwood rule, it probably won't take long; boxwood is pretty dense/not porous.