Up here In Toronto we have a good product called Krown rust control,
it is a thin oil that they spray all thru the inside of all the panels and the underside.
I have used it on my 89 Dodge van and it still has no rust.
The good thing about the thin oil is that it creeps into all the seams of the metal where water likes to get into and never dry and starts to rust.
The oil will prevent that from happening.
If the oil or rust treatment is too thick it will only create a bridge and in the cold will get harder and water can sit under it and cause rust.
I have seen that many times on cars where it rusted under the waxy spray stuff.
Try to find out if they have any dealers in the states or look for thin over thick.
Don't use old motor oil.