mineral oil based vehicle rust inhibitor

I've owned three pickup trucks over the past thirty years or so and they all ran great but died of frame failure. (well... the Chevy had cylinder head trouble and a rusted out frame)

I did some googling on the topic and saw that folks were spraying on rust inhibitor products, old motor oil, ATF. WD-40....

I found a product called NH Oil Undercoating that was essentially a mineral oil based product with some chemical additives. I looked into mineral oil some and found it wasn't harmful/corrosive to plastic and rubber and so I mixed up a 90/10 ratio of food grade mineral oil (about $5. / 16oz. container) with some mineral spirits and put it in a garden sprayer. I adjusted the nozzle and went to town on the undercarriage, inside the doors, tailgate, rocker panels..... After 6 months there is still a visible oil film on the frame rails and cross members inside and out.

This seems like a pretty inexpensive way to beat the rust. Your thoughts please.

Good luck is all I can add. Here in Vermont they spray a brine "to keep the roads safe, but not up to normal speeds". What it really does is save the state a ton of money initially compared to salt I guess but it needs to be used more often. It also becomes an icy nightmare the minute the roads don't get plowed off and the temp drops. Then the genii bring out graders to clear Rt 4 because it has 4" of ice buildup.

The other thing it does is absolutely kills equipment. 5 year old state trucks look like they are 20 years old. So we save money...until it comes time to upgrade equipment at twice the old rate. We are ruled by dummies.