I have used boiled linseed oil with great success on the underside of several vehicles. I did my Dodge pickup last Sept, and it still looks perfect. I paid to have my bus Waxoyled by a body shop some years ago. I was okay for a year or two, but it eventually all washed off in exposed areas. I like to spray Fluid Film into the door bottoms and all cavities on all my vehicles. It creeps like crazy and seems to stay put for a long time. the only disadvantage of it is that it eventually creeps out the rain holes in the door, and leaves a greasy black strip along the bottom of the door, although it can be wiped off easily.