I made the mistake of spraying diluted bulk engine oil to the underside of my 2 year old '64 one ton DRW flatbed Jimmy. The truck was used to pull trailers in and out of unpaved work sites. Man what a mess with all the dirt mixed with oil.
Doing any kind of maintenance under side would result in lots of black on my cloths.
After a couple of times under side I took the truck by a steam cleaner. I sure wouldn't recommend spraying oil under a vehicle if its traveling dirt/gravel roads.
We have about 6 miles of gravel road getting out in one direction. I've found under coating eliminates rust issues/noise to the body and frame. This part of the country isn't in the salt belt.