The coating is cosmoline: it is used to keep metal in long-term storage from rusting and corroding. Your rifle should have been generously coated in it, from top to bottom. The good news is, the metal is in great shape... the bad news is, it ALL has to be removed. Not a real major task, but you will want to clean the bore and reciever several times using a good penetrating oil and solvent. You will also notice that the wood will need a good cleaning as well, since the general task was to grease it up completely, regardless of whether it was wood or metal.
A few hours and you should be ready to hit the range!
As for the barrel, you may want to get a plug for the end of the barrel, pour in solvent or a combo penetrating oil/bore cleaner, and let the barrel soak for a day. Then go after it with a brass bristle brush. That will remove all the rust, cosmoline, and any traces of brass from former shooting. Once your patches come out pretty clean, you will know you are done!
Enjoy the rifle: it should be a blast to shoot for generations to come!