Years ago, I was given a Marlin Model 1892 lever-action rifle in about the same condition.
The only thing I did was to take it apart, carefully with proper-sized screwdrivers, and scrubbed all surfaces in hot, soapy water with a nylon brush.
A few interior parts had light rust, so I took a brass brush to them, since they wouldn't be visible.
Rinsing and drying all the parts, I let it sit a week in my warm apartment.
I oiled the metal parts lightly with 3-In-1 oil and reassembled.
The wood, I washed quickly with hot, soapy water and scrubbed off the grime with a soft, nylon brush. All I wanted to do was loosen and float away dirt, but not finish. Worked very well.
After the stock and forearm had dried for a week, I oiled them lightly with pure lemon oil. NOT Pledge or any polish that contains wax, just a simple, small bottle of pure lemon wax.
It soaked into the wood beautifully but didn't give it an artificial shine.
The rifle still looks original, but I feel better knowing that the crud in the nooks and crannies is gone and can't soak up moisture to promote rust. I've had it about 15 years now, and it still has its brownish metal, with slight patina.
So ... did I do wrong? Is removing grime and dirt from an old gun a big no-no? I don't believe so, unless that crud has historical value such as an old rifle used in the trenches of World War I that still has splatters of mud on it.
In this instance, I believe removing the crud would be sacrilege.
But how do the rest of you feel? Will a quick bath in hot, soapy water and light scrubbing with a soft, plastic brush affect value?