I'd stay away from Vasaline and from solvent cleaners such as Hoppe's. Most knife collectors use some form of paste wax. The best is Renaissance Wax (also called Ren Wax) which is a museum-grade preservative wax. It's non-abrasive, non-reactive, and safe to use on all materials, including steel, brass, stag and leather. However, it's also expensive and can only be bought from specialty suppliers. If you only have a couple of knives, I suggest you use a good quality, non-abrasive paste car wax.