I have 100's of fence posts that have been in the ground 25 years and learned from my grandpa who has 100's of that have been in the ground 60+ years . Dig your hole at least 1/3 the length of the post , more of it will carry a load. Don't fill the hole with the dirt use 5/8ths - or 3/4- to fill your post holes . Using the minus gravel with the fines allows you to pack it down tight. The gravel keeps the water away from the post better than the dirt. Grandpa used to also soak his post in motor oil in a 50 gallon drum.