When I was buying milsurp rifles by the dozen I used straight mineral spirits to clean off the years of cosmo and grime.
I built a tank using 8" PVC pipe about 5' long with both ends capped, then the pipe was cut length-wise a little above the half way point. The top piece was hinged to the bottom for a lid.
Filthy rifles were put in and soaked in mineral spirits for a while then scrubbed with a parts brush.
They come out squeaky clean but a little dry.
Currently I use Ed's Red in a stock pot for handguns. The grips are removed and the gun is field stripped and submerged in the Ed's Red, then a tooth brush is used.
The Ed's Red has mineral spirits in it as well as other fluids to lubricate the inards.
The mixture will dry but leave everything lubed.
So......
The pure mineral spirits will clean up the metal without hurting the finish but might leave the inside dry and unprotected.
RDave