I use cedar for posts, Eastern Red Cedar. I can show you posts that have been in the ground for 70 years, and are still sound. Although I have used them as soon as cut them, I prefer to cut them in the winter when the sap is down, and let them dry for a few months. The redder the post is, the longer it will last.
Yep. The less sapwood (white wood) the better. You want the post to have as much heartwood (red wood) as you can get,
yes, and the same goes for black locust as well.
for poor folks w/a local supply, slow growth post oak & other members of the white oak family will last a surprising long time. the heartwood is what holds up and lasts. in the white oaks, the wood cell pores are occluded, meaning they are full of minerals. in the red oak side of the family, the pores are open, vastly reducing service life.
black locust is far better, but given the difference in prices, well, an astute small farmer always went with the post oaks collected from rocky hillsides with thin soil.
