I didn't think Creosote was readily available anymore due to it being a carcinogenic. If you can get it then mix with old engine oil and soak the timber. My late father and uncle used this method and the fence panels uncle treated 50+ years ago are still good. Every few years he lays them on a sheet of plastic and soaks for a day or so, just to get the colour back.
Better still is probably transformer oil, but I do not know where you would get it from. Father used to work for the CEGB [electricity supply] and when they stripped the grey box electrical transmission transformers you have at the side of the road the crew used to nearly fight over the contents. The cooling oil used to soak deep into any timber and the plywood insulation panels they pulled out were much prized for garden shed bases etc.