You using hard or soft wood?
Back in the day you could buy coal tar creosote by the 50 gallon drum. Worked great. Then along came the EPA and banned the commercial sale.
I have used tar sealer, like you put on a driveway and it works pretty good.
If you have time to soak the posts or boards diesel anti gonna hurt it. Might do the trick.
I’ve been searching for just a small amount of creosote for years. Back in the day it was my favorite aftershave.