If im not planning to shoot it and the gunk isnt causing further degradation I leave the gunk. Part of the look. I call it patina though to try and legitimize it.

If you take a dusty barn find GTO on a trailer and set it next to the same car on the same trailer- but washed- you will get more money for the dusty barn find. Same is true for antique guns. If the grime looks like it got there through years of use you are only cleaning off the history of the gun which is where the real value is. In the vast majority of cases most people cause more harm cleaning an antique gun. Just taking a butt stock off and turning screws out changes the look of a gun that has had the seams between metal and wood fused by the grime of use and time. I have heard every argument under the sun from both the “i always clean my guns real good” folks and the “i dont touch one spec of that hard earned rust” people. It probably doesnt hurt much to gently rub the wood and metal with a little olive oil on a piece of canvas if you just have to do something. Overall unless leaving the gunk of time on the gun is causing it to actively decay in front of you I would leave it alone. And whatever you do dont scrape the rust off with a penny. It looks like crap when people do that.