More than 25 years ago I fitted a new fence using concrete posts and gravel boards.The whole fence including the concrete was covered in Thompsons Waterseal ... everything is still in situ. including the larch lap fence panels I fitted at the time.
The panels were painted on both sides, with old engine oil each summer for the first few years, and after that I have used whatever was available at the time ... usually cheap old Fenceguard. The panels are rock solid, with no signs of rot, I'm sure they will last forever.
Whereas I fitted the exact same fence for my neighbour about 8 years ago. These panels were only treated on my side and he left his side to the elements. They needed replacing last year as they had warped and rotted through, and the wind soon demolished them.