Do not use used engine oil. Over time will ruin anything rubber. Instead go to the local tractor supply store and buy a gallon of tractor hydraulic oil. Not tractor transmission oil. Tractor oil won't swell the rubber parts on your truck like door seals and brake hoses.
Then buy an Eastwood undercoating gun. It's around $60.
Spray every nook and cranny and leave it sit for a day so it can soak in. I do this every 1-2 years.
I have a 1996 F150 that has zero rust and live in Ohio. Now I don't drive it much in the winter time because its 2wd.