It’s wood. Use oil-based wood stain.

I bought deck stain at the lumber yard and put it on with a roller on a warm day.
Bonus: it looks good too.

Not a fan of TWS. Don’t find it to be what it once was - changed formula?

I’ll bet used motor oil will preserve well, but lots of downsides to it.

If you take the time it takes, it takes less time.
--Pat Parelli

American by birth; Alaskan by choice.
--ironbender