Motor oil is used as a wood preservative for fence posts. Untreated wooden fence posts are susceptible to water damage. Water damage makes the wood vulnerable to termites. Termite infestations quickly cut channels into the wood, reducing it to powder and leaving only the severely weakened cellulose stalks in the wood. Motor oil kills termites that come into contact with it, and repels those who come near it; and the oil repels water to protect the wood from water damage.
Spread the tarp over a flat, relatively dry surface. Put the bag of cat litter on one corner and the open 55-gallon drum at the opposite corner, as if they were boxing opponents.
Transfer the 20 gallons of used motor oil to the 55-gallon drum, working over the tarp. Use the cat litter to dust over any oil spills on the tarp.
Lay the two 2-by-4s parallel on the tarp. Spread them apart a little less than the length of your fence posts.
Don the rubber gloves and plunge one end of the fence post into the 55-gallon drum. If the oil doesn't cover at least one third of the length of the post, use the plastic cup to baste the post with oil to cover at least one third. Always work over the tarp.
Take the post out of the barrel and lay it across the 2-by-4s, using the 2-by-4s as a drain rack. Cast some cat litter under the fence posts where they drip, to prevent oil runoff from the tarp into the local soil.
Sweep the oil-soaked cat litter into a bag, and mark it "Petroleum Waste." Take it to your local recycling centre for disposal.
Motor oil is toxic to a number of organisms. The greatest danger of the oil spreading from the fence posts is not seepage, but floating. During rains, the oil floats away and migrates into surrounding soils. Good drainage around the fence is essential; and mixing sand with the backfill for the fence posts will decrease the runoff.