a guy in town does oil undercoating, but i don't know what kind he uses.
i have been oiling my "bus" [99 caravan] since i acquired it in 2014. it came from arazona, so it had almost zero rust except where the paint was knocked off.
i drilled extra holes in the doors and gate, plus drilled holes just above the foam they put in from the factory on the dog legs. also popped the vent holes and other caps on the inside of the rockers, then saturated the foam with oil.
pulled the tail lights and blasted the inside quarter panels good as well.
just about anywhere the factory used foam was saturated with oil, and any place there was a body plug of sorts got the spray as well.
i do this just before the weather turns cold, but that stuff leaks EVERYWHERE when the temperature gets above 60 or so.
so far, i think this will keep this thing going until i croak.
i also took the spare tire out from underneath and tossed it in the back. i did this for two reasons. 1 : oil the floor pan, plus the spare and the crank mechanism can't collect anything that would cause rust. and 2 : easier for my creaky bonzes to get the spare out of the back than from underneath.