Dry Wheels
As I have posited elsewhere different timbers need different approaches. Oak generally does not need anything doing to it either internally or externally as it is pretty robust. However softwood is vulnerable to the weather and to vermin. Whilst oak can normally only be attacked by Deathwatch Beetles, softwood can be attacked by a whole range of wood boring insects.
In the building industry the general approach is to keep softwood timber dry and to treat it against woodboring insects. Providing this approach is followed rigorously, it works very well.
My only reservation is that the wheels, stays, sliders etc: are of small section wood and may be more vulnerable as a result. Regular treatment with wood preservative seems to me to be sensible.