The best bet for what you have described is cheap or even used motor oil. Linseed
is OK but would cost more I suppose. Why not white oak which is more plentiful and works as well? O/S used 1/18" x 12" laminated edgegrain larch for many, many years as a platform floor material. I've also seen them use 2" nominal YP which is a great floor material. Waterproof treatments like Thompson's, etc. must be used every year and are expensive. No good for this usage.
