Oil, ATF and fluid film will most likely wash off after a time. Many rustproof materials were meant to adhere to the underside for years, but inside doors and other places would remain soft and sticky. The soft sticky material wasn't used underneath for washing away was a real issue.
If it's done right it should help but many manufacturers do a good job and having holes drilled causing broken paint and undercoats to be damaged messes that idea up. Best to have the interior trim panels removed for gaining access to inside of doors, splashliners removed to access fender openings, etc. In other words lots of work. ($$$)
Also best to do when the vehicle is new, you don't want to seal in moisture, dirt and mud as it will never dry out and has only one way to go, thru metal.
Subframes and suspension parts are bad as they don't clean the surface after welding brackets and such, just prime or paint over them. The first place you see rust on those parts are where the have been welded and that is why.
A friend has a Toyota pick up in Vt, lots of salt used there. He was adamant about cleaning the underside regularly after storm clean up and used this vehicle daily. When Toyota had the recall for bad, rusted frames he brought his in and they didn't replace it as it was solid due to the extra time he spent cleaning it.
My boss has a mid 80's Wrangler for plowing, built from a couple wrecked units. When it was put together he had the frame sandblasted as best he could and epoxy primed and painted. He washes it after a storm and will do so if it storms a few times a week. About once a month it goes on the lift and gets crusty spots scraped, either a rust conversion paint touch up or he used to use old axle joint grease applied with a brush. It seemed to work as it's been plowing for over 30 years and is now a back up to a newer Jeep.
So, I guess it comes down to being pro active on maintenance and rust removal on a regular basis for something you wish to keep in the family especially now that 4wd trucks can cost as much as a small house. Or even a decent car for that matter.