Your blade, being steel, will appreciate regular coats of a clear, non-organic, oil. This prevents moisture from reaching the blade, as well as giving a nasty sliming to anyone who touches it. Collectors of Japanese swords recommend choji, which is mineral oil with a small amount of clove oil for scent. We also recommend a light oil such as sewing machine oil or gun oil. We've heard of using 10W30, but can't comment as its usefulness. Oil should be changed regularly by wiping the blade with a soft cloth until dry, then applying more.
Cosmoline, a vaseline like substance, and grease were often used in the past, but now are getting looked down upon. If you have an old blade covered in it, rest assured that it will still be protected, but there may be microscopic pits in the blade that would make a collector of Japanese swords cringe. The stuff is quite ugly as well, but at least it keeps hands off....
Another option is to apply a wax coating. We've heard of using carnauba wax (like on your car), but cannot comment to its long term effects. You might want to check your local automotive store/department to ask about which waxes are best on steel.
The final viable option is a clear varnish. Del Tin, a respected maker of medieval replica swords, puts a coat of varnish on their sword blades. We recommend making sure there are no fingerprints or specks of dust on the blade first, or else they will eat into the blade from beneath the coating.
Handle and Scabbard Coatings
We still don't know of any accepted long-term protective coatings for leather scabbards and handles. Any help would be appreciated.