Nothing but paint will protect for any extended period of time. If you are willing to go to the periodic renewal requirements route a good marine Spar varinish is your best bet.
Eppiphanes is the best but the price will absolutely floor you.. Interlux and several other brands are good for the money but you will renew it more often.
I built the exterior of my house out of black walnut timbers and trim so I’ve looked and read extensively..