I've tried the used oil, (dirty and smelly) the new oil, (drips off over time) the commercial stuff (expensive). They all work, but have there drawbacks as noted. Here is my preference, and it is time tested to prevent rust LONG TERM.

Get a gallon can of wd 40. Mix in 2 cans of STP Oil Treatment, and stir it really good until it is dissolved in the wd40. Put it in an old paint sprayer. Spray it in the doors, inside the door posts, inside the fenders and quarters, rockers, tailgate, box braces, between the box and the cab, above the wheel wells, gas door, backside of bumpers, bumper braces, inside the hood bracing, door hinge areas, the entire bottom side, axles, driveshaft, suspension and springs, get it everywhere except the outer finish. Whatever overspray gets on the finish wipes off easy with no damage. It take an hour and cost less than $25

The wd flashes off over time, and leaves a very thin coat of STP, that clings tenaciously thru rain and snow and car washes. But wipes off with mineral spirits if you ever need to. A re-spray every 5 years or so ensures no rust for ever.