IMO there really is no way to protect lithographed cans outside. Nature of the beast is to fade and rust away. If you want to maintain them in collectible condition, keep them inside, out of direct sunlight especially. If you want to display cans of some sort, I would suggest buying some of the fantasy containers where they are painted with auto grade paints, then a decal or two applied for the branding. That way no collectible cans will be ruined.
On a side note, if that Mobilgrease were in my collection, it would not be outside, it looks pretty nice!

Darin