If timber needs to be poisoned (the more realistic term for "treating") then it shouldn't be used at all, use an alternative material.
Fencing posts are "treated" with the aid of fire since prehistoric times. And the charred part of it is all we see today - surviving thousands of years in wet ground.
But when one thinks of it-isn't creosote a form of charred/destilled cellulose itself I wonder....
Visit a historic ring village and ask the tourist guide for information. All the reminent timber that was found is charred. Unless it was burried in peat, but that is a different story.
So burn the pole ends 'till they are pointed, covered in charcoal that goes into the ground and leave the rest in a natural, rought sawn state. No paint!