I've had my truck for 20 years, no undercoating or oil. Just make sure you rinse it good underneath if the weather warms up during the winter and then again in the spring. I've had some break lines rust out and started to get some bubbling around the fenders, which I had fixed. Don't know if undercoating or oil helps, have heard mixed reviews. Here she is.
I get all my trucks and cars undercoated at Elliott's Undercar in Butler. It's not the rubber stuff and not the oil, more like a thick sticky grease. What he uses and the way he does it works great. Been doing me stuff years with good results. Had him do several used trucks also.
I have noticed a lot of the newer trucks are rusting over the rear wheels... I was told one of the three manufacturers are using a chemical on the frames that inhibits rust sprayed on during the manufacturing process... A lot of body shops are not fixing rust anymore because very little profit most are doing insurance repair which has higher profits...
Been using it for years after a chemical engineer and hopelessly crazed car nut turned me onto it.
I detail my vehicles top to bottom two times a year, in the spring after winter and in the fall before. Spring cleaning my truck involves pressure washing everything I can see and reach underneath, the frame, floors, suspension, back side of the rear chromed bumper, wheel wells, skid plates and etc. I even drop the spare tire carrier so I can clean it, it's carrier and the bottom of the bed. After Fluid Film applied on everything since the pressure washer/detergent removed most of if not all of the previous coating along with the road crud.
FF doesn't dry as such, it remains kinda gooey, collects road grim but most importantly it provides a barrier between all parts and the nasty road stuff. Oh, and it doesn't need to be applied an inch thick, a light coating will do. Last June when I had my truck inspected checking things out ..."Dang, this thing looks like brand new up here!"