I had about 10 surplus Bren Mags caked in Cosmoline, so I just disassembled them, put the in a garbage bag, and left them in the sun for an hour, and it simply wiped right off. I have heard of people using a variety of methods such as dishwasher ( wife would not be keen on that) , boiling water, etc.. but this worked well..
before and after..
before and after..