We have a large property and I use the spent oil to help preserve around the bottom of fence posts. I dig a small trench around the bottom of each post and pout the oil into it. It usually soaks into the wood of the post and to some extent, the earth surrounding each one treated.
I have never found the oil creating a problem for the surrounding earth the whole time I have been doing it, and in fact the oil appears to dissipate quite quickly into the ground and causes no harm to anything that I have noticed.
I mostly do my own oil changes, so get to keep that oil for myself, but spent oil is now saved by auto garages when they perform oil changes on vehicles, then it's picked up to be reprocessed by those who see an easy cash cow in doing so - in other words - making good money from a substance they've been given for nothing. They then charge you a mint for your own oil back again when it's once more refined and all the impurities are filtered out.
I recall pulling up at a garage many years ago to fill up with petrol and noticed through a slightly opened door to a workshop area, a man, apparently trying to conceal himself behind the door, filling up various sized bottles of oil (pints and quarts) from a plastic container which I recognized by the label as a much cheaper brand of oil obtained by the method described above.
The bottles being filled were sitting in a wire rack advertising Shell XMO. So instead of the oil in the bottles being unadulterated and top quality - as advertised, the oil in them was now a cheap substitute! This 'cheap' oil was obviously being prepared to be placed next to the gas pumps with the price being charged commensurate with the Shell XMO label on the rack.
Never pulled up to purchase anything from that garage since!