Can most any timber be used for weather exposed situations
providing it is well treated with preservatives?

This is something I have been wondering as well.
The barge board on my sisters 12 year old extension is completely
rotted away.
The woodwork on our 100 year old house is slightly rotted in some
exposed places.
Teak never seems to rot.
When you pick up a piece of wood, how do you know how resistant to rot
it is?

I don't know the answer but I would never use kiln dried wood outside
however much preservative was applied to it.