We use these self tapping screws on our trailer floors.
http://www.imperialsupplies.com/items.asp?item=0692940 We start out by drilling a 17/64" hole through the wood and the cross member. We then have a bit that drills the wood to 5/16" and countersinks it. This can also be accomplished by drilling a 5/16" hole in the wood and using 1/2" bit to make the countersink. Our cross members are tempered steel and we have been repairing floors for 40 years. I would have fears that the 1/4" decking screws you see at bumper pull trailer dealers are made to screw into cross members that are barely thicker than a beer can. The floor screws I describe can be bought at any dealer that sells semi trailers.
We don't put anything on our floors in our dry vans. On flatbeds, I still like mopping linseed oil on on a hot day.
