I've washed them out with dawn dish washing soap, though I didn't use that much! We filled them about 1/3 of the way and then rolled them around for about 30 minutes.

Rinsed them the same way 3 times and then stood them up and filled them to over flowing. Soap and oil are lighter than water so they will float to the top and out thru the bung hole.

Turn them upside down and spray the inside for several minutes and let the water run out. It will bring the balance of the oil out.