DO NOT USE WD-40!!! Firearms are not car parts, rusty bolts, or stuck pieces of farm machinery. It is corrosive and it will eat whatever finish is left on your Mosin Nagant. Trust me.
Your best bet is to use an actual gun cleaner/degreaser. Some people will use a brake cleaner, but if you do make sure you clean it well. Then, I highly recommend "Break-Free" brand CLP for everything there on out. The military guys will know what I am talking about. This stuff is awesome and it does it all (cleans, lubes, preserves). I use it on all my firearms and have done so since about 1996.
Never use grease of any sort. A light coat of oil (CLP) is all you'll ever need.