I had first suggested Brazilian Wax, which I heard did a nice job, but I didn't see any available in the gun catalogs, so I deleted the post. I never tried it myself.

I use Bri Wax, its a thin furniture wax. I tried the Formby's, Tung Oil, lacquers, etc and they filled in the fine engraving, yellowed, and I had a hard time getting it out. So for me it didn't work very well.