The oil and diesel is the cost effective way to treat the boards.
Even on old boards, the mixture gets sucked in faster.
When the boards are new, linseed oil, though expensive gets the job done. When the boards are 5 years old then the diesel and oil works way better.
If you have a lift or pallet forks, raise the trailer upwards, then coat the boards beneath too. Down south wood bees will destroy these boards from the underside.
The expensive way is to use the garage floor epoxy paint. No need to add the glitter. Gray works well for color. Adding a little black, shaken at the paint store will darken it.