As someone who owns a paintshop I will chime in...
There are a few things you need to know to help you in the process.
1. POR 15 is not a great solution, all it does is encapsulate the metal forming a protective barrier that will peel, crack, fail sometime down the road. Of the several restorations I have done for folks over the years, many have used this product on frames, floors, etc... all of them without exception have failed between one and six years down the road. Does it work for a while, SURE, but it is not a fix... just a band-aid. The best thing that you can put on freshly blasted, clean metal is Epoxy Primer. Epoxy bites to almost any substrate extremely well, and has good durability. If I was trying to ensure that a rust-free vehicle remained that way I would prep my surface, shoot epoxy, shoot paint/clear and stay on top of cleaning the undercarriage frequently to ensure moisture/mud did not have an opportunity to sit and rot the metal away.
2. Most of your body rust is not going to come from from the underneath... Most panels rust due to poor drainage from the inside (ie; inner fender wells, bottom of tailgates, cab corners). No matter what you do to the outside of these panels, there is not way for you to apply something to the outside to protect the inside
The factory uses seam sealer, foam, and other types of material between inner and out fender wells. They do this to cut down on noise, moisture build-up... the problem is that once these materials begin to fail, rust eats away the panel quickly. The best way to ensure that outer body panels don't rust away is to ensure that drain holes are always kept clear, and to apply something like body-schlutz, or another oil or waxed based spray to the inside of the panels in order to wick moisture away and keep it away. Make sure and clean out the inside of your cab corners a few times a year, and it can't hurt to sand them to shiny metal (with around 220 grit), and give them a few coats of paint so that they are protected.
The advise you have been given to spray down the metal with oil is actually a very good process. Now, as someone who does paint work I hate to advise someone to do this, as in my line of work oil/silicone is the devil... but it sure does help to keep rust away (as long as it is applied fairly regularly and not allowed to cake up with dirt/grime and become something that ultimately traps moisture.
Ultimately... as much as you want the metal of your truck to last forever, it just cant. Metal has a life span just like everything else exposed to the elements. Whatever you do, please don't use something like bedliner or the like... it will do its job for several years if done right... but when it fails it will trap moisture like nothing else
Then it's bye-bye sheet metal.
If you have any specific questions please let me know... take care of your truck and it will take care of you.