I hadn't heard of tranny fluid! Makes sense. I can see where the wood preserver/sparureathane combo would give a great finish in addition to protection,too.
I treated half my privacy fence with linseed oil and half with Thompson's Water Seal (as seen on TV) because the commercials were good and never seen it used.
The linseed oil is good, the water seal didn't last a season. I wouldn't use linseed on a deck, though, because of the residue that my fence had for quite a while.