It is not a good idea to put used motor oil in any contact with the ground. It will wind up in the water table where it will pollute incredible amounts of water. You'll also eventually drink it. I recall a study from Maine that a very small amount ( less than an oz?) of used motor oil would pollute around 10,000 gallons of water. Other things I read said 10 times that amount.
Now, I'm no tree hugger and have no problem with most things but don't want to drink used motor oil with all the fluids and heavy metals it contains. It's OK if others want to do it but do it straight from the container. In water it also is consumed by animals that we depend on from critters and birds to dogs, fish and farm animals. I like to fish.
Rethink used motor oil for any application where it can contact the ground. There are suitable substitutes--still oils--that work just the same and don't cause the drinking water issue.