I've sprayed the inside of the doors, inside of fenders and over the wheel wells on my 90 Dodge truck every spring & fall since it was new. I am the original owner. It doesn't get driven much anymore, but it used to go out daily as it was my only transportation until 2006. It has over 200K miles on it right now and NO RUST anywhere.
When it was about 15 years old the power door lock motor needed to be replaced in the drivers door, and when I removed the door panel there was a layor of this thick grease looking stuff in the bottom of the door from all the coats of WD40 I had sprayed in through the drain holes. I still spray it down twice a year with the stuff. I use a whole can in the spring (after a full rinse with a garden hose) and a whole can in the fall. Works great, it's easy to apply and the stuff is cheap.