Used oil soap

For having soap to clean clothes from used oil, mix 500 g of caustic soda with 3 litres of water. Immediately, add 3 litres of well strained used oil. Mix well. During some hours, mix from time to time until having an smooth mixture. Add a glass of fabric softener during one of the mix time. Put in a big cast. When it gets solid,  cut with a knife and let pieces during a month to be dried. All process must be done in a very well aired place and with protection.