Spray oil underneath truck

That oil is going to make your problem worse IMO.
Try something like this.
LINK

Since that truck is already 8 years old, you don't have much un-rusted bare metal under there. Any oil sprayed underneath will either coat the already-dirty parts and not do any good (and will just attract more dirt), or coat the already oxidized/rusty parts and not do any good. Petroleum products that come in contact with rubber parts will do harm. That would include the short brake hoses near the calipers, the short fuel line connector at the gas tank, exhaust hangers, etc.

I use Fluid Film on all my trucks and farm equipment. Like it much better than the oil/fuel mix.

carringb wrote:
Rubberized vehicle undercoating will be cleaner to apply and will last longer. It also suppresses the noise from gravel hitting the underbody.

Around here, using oil outside for anything (including dust control or rust-proofing) would be a good way to get get fined by the DEQ. And oil sheens make it easy to find an offender.

^^^^^^ This.

Bondo makes a great rubberized undercoating. I have used it before and it goes on really nice.

we used to use diesel on the underbellies of the plow trucks, a little went a long way. Rubberized coatings are great if they completely seal, otherwise they just hid the rust.

Not too worried about regulatory agencies. Just want to know if it works.

Rubberized vehicle undercoating will be cleaner to apply and will last longer. It also suppresses the noise from gravel hitting the underbody.

Around here, using oil outside for anything (including dust control or rust-proofing) would be a good way to get get fined by the DEQ. And oil sheens make it easy to find an offender.

I wouldn't do it, you will only attract dust/dirty which in turn will attract moisture, then more dirt etc. Every veh will get rust on the frame even those that have never seen dirt roads

If it is bare metal, up North, I would get it "Zebart" undercoating. Spraying oil will just creat an EPA issue.