I would field strip it, wipe off as much cosmoline as possable, use a tooth brush and apply as much solvent to the internal workings as possable(soak it) i use hoppes no.9
wipe this off, then submerge the gun (field stripped) in a bucket of hot(tap water hot) water saturated with dawn dishsoap(great de-greaser) work the trigger under the water about 20 times double and single action, take out and rinse.
after dry, apply gun grease to the slide and working parts, and a light coat of oil on everything els,
I get a lot of surplus guns, mostly mausers, and this seems to work to get the gunk off and make em work best.