i get my 99 caravan [my "bus"] sprayed every other year by my one buddy that has a small repair shop.
he has a "secret" [] concoction of oils he uses, but it's not black. it's redish, but there is no transmission fluid in the mix.
i drilled a bunch of holes in the doors, gate, dog legs, and rockers, so as to saturate the foam stuff used in these areas with oil. these things really rust around here due to the salt brine mix used in the winter. as this thing came from arizona, and only had rust specks due to chipped paint, i want to keep it this way !
his "formula" seems to be working, as it's still the same as it was when we got it in 2014.