[lugerholsterrepair]

Matt..The frame rails could indeed be rust welded together but I have to wonder. I have taken apart 2 different rust balls. And I MEAN pistols you would think would be beyond salvage. I had to soak them in Kroil and hammer them apart.
Oddly while the outsides were horribly rusted the insides were not!

To answer your question..any good penetrating oil should do the trick. I used Kroil but I never thought it was all that great. No general purpose lubricant will affect the bluing or straw if there is any. In fact a lot of oil and 0000 steel wool will remove light rust without hurting it either.

Just let er soak untill it begins to wiggle. Patience helps. You want to be sure the takedown lever is in the proper position.
I wouldn't hesitate to tap lightly here and there with a plastic or rawhide mallet. Tapping sometimes is the catalyst to getting things broken loose.

From my experience though, I would look at something mechanical while it's soaking.