Best wood protection for underside of old box truck?

I own a 14 year old box truck with a standard original wood floor (supported by steel beams every 12 inches, bolted in) and aftermarket carpet above inside the box. I am not sure what kind of wood it is but it feels dense. Over the years, some of the wood around the rear wheels has rotted and fell way due to water from the road being kicked up while driving (UV is NOT an issue here). I recently replaced the rotted areas with pressure-treated pine (1.25 inch deck wood from my local Home Depot) as it most closely matched the original wood's thickness and was inexpensive. The remaining original wood is in fair to good shape but the underside is very dirty (almost black)from road oils, dirt, etc being driven into it over the years.
Question: What is the best way to protect the remaining wood? I had first considered a "shield" approach - an underside elastomeric coating as it repels water and is non-flammable. However, upon further research I realized that might actually accelerate wood rot if water found its way into the wood somehow and became trapped (from around the truck sides, seams, from above, etc).
My best theory now is a "wood penetrating" approach: First, clean the underside with a mild detergent and, after complete drying, apply a deep-penetrating, professional grade, oil-based wood stain (as the original wood has already had years of oily undersurface exposure). Armstrong Clark and Ready Seal are on my list among others.
Either way, any recommended basic approach? Brand names? Suggestions? Again, UV is NOT an issue, nor is appearance. I just want the most, best, longest lasting wood penetration and protection, right? By the way I am a novice handyman at best. Thanks for any help!