we used a lot of WD-40 on guns growing up, that and Hoppe's #9, but i myself have found for firearms there are just way better products available.
cleaning and oiling are the first lines of defence against rust on your firearms. if you have many in your collection, then periodic checking should be done from time to time to see if there is any rust. because the quicker it's found, the faster and easier it will be stop it before it does permanent damage.
gun cases, (most) are for transport and short term storage only. i have seen what long term storage does to a firearm, it's not pretty. if you take a firearm out after long term storage, might be a good idea to do a proper cleaning and oiling before shooting it. also check the bore for obstructions.
the brand of oil is probably not near as important as the fact of using it. one oil i found from a Lucas rep, is their gun oil, red in color and lightweight. another oil i use is RemOil, probably works as good as any other. but use it, it's what keeps them from rusting. i have seen many guns turned into scrap iron because of rust and seen many that were in almost perfect condition after decades of use.