Congrats on the new arrival -- FWIW, I haven't found that new mags are actually cosmoline, just grease. These mags are aluminum so scalding hot water, dish soap, and a bottle brush, are fairly good at cleaning. I'd recommend dumping down the toilet, not tub drain though. And rinse again w/ scalding water. For steel actual cosmo mags, I use a light oil like wd40 or rem-oil after drying but not essential on these aluminum mags so long as you let them dry.
Again, congrats on the acquisition.