Look in the curio and relic discussion subforum, there is a sticky called mosin cleanup. Has everything you need to know.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
Cosmoline is essentially a grease. Grease is oil mixed with, or suspended in, a binder - usually just soap. So anything that cuts, dissolves, or loosens oil (grease) will do it.
Very light oils at first...then soaps.