OK, this sounds crazy, but I had a vision.

Have you ever tried buying creosote for posts to keep them from rotting?

It's been banned.

You can buy the wood preservative, but nothing was as good as that creosote.

Remember the smell of the railroad ties as I rode my dirt bike down the tracks as a kid?

OK, so changing oil yesterday, the oil from my truck came out black as coal, and I over-maintain my vehicle.

I had a chunk of treated 2X6 trimming and spilled some of the drained oil on it as I was making my way to my recycle 5-gallon jug, and it soaked up that black-as-coal oil so fast and it hit me; I can use my old waste oil as a wood preservative.

I wouldn't put it on a deck, but any part of the wood buried underground.

Take a Home Depot bucket, dip the fence post in it, and soak overnight.

Bet it would work well.

Anyway, thinking out loud.