Used motor oil, when used as a wood preservative, can extend the normal life of a fence post by 15 to 40 years. The type of wood the post is made of is a determining factor as to how long the post will last. Recycle used motor oil to cure fence posts by soaking them in a barrel filled with oil for a period of time. The parts of the fence that are treated with the oil will be buried underground. Set the posts in concrete to avoid contaminating the soil and groundwater with oil.
Lay the 247.5 litre (55 gallon) barrel on its side and cut the top of the barrel off with a hacksaw or reciprocating saw.
Set the barrel on the ground in the area where you want to treat the posts. Draw a line around the bottom of the barrel in the soil using the handle of the shovel. Remove the barrel from the circle.
Dig down 60 cm (2 feet) into the ground using the circle as a guide. The hole must have a flat bottom to accommodate the flat bottom of the barrel.
Set the barrel in the hole.
Fill the barrel with 112.5 litres (25 gallons) of used motor oil.
Lay a post on the ground and incise the section of the post which will be soaked in the oil with an incising hammer. These can be found in the tool section of most home improvement shops. The hammer has spikes on it and you pound the spikes into the wood every 7.5 cm (3 inches). This allows the oil to penetrate the post.
Drop as many posts as will fit into the barrel with the incised end down in the oil. Allow the posts to soak for as long as you'd like, but a minimum of seven days is recommended.
Allow fence posts to cure for 12 months before treating. If the posts are still green, they need to dry out before being treated with oil. This process is called seasoning and it allows the wood to dry out so that splits and checks open in the wood. This allows for increased preservative penetration. Just lay the posts out in the weather, but keep them 45 cm (18 inches) off the ground. Ask local auto-repair or oil-change centres for some used motor oil. You can usually get it free for the taking. Put the lid back on the barrel and continue recycling the oil to treat new fence posts.
Don't allow oil to spill around water sources. Encase treated posts in concrete to ensure that the oil doesn't leach into the ground and groundwater. It will also increase the lifespan of the posts.