Don't try cooking oil. Gums up terrible, toss the pump and the hoses, but the fittings come clean.
Peanut oil works ok. Have a friend who works on drilling rigs offshore. He brought us several 5 gallon buckets of it that was used in a fryer. We thinned it, strained it, and put it back up in buckets. It seems to coat the hull and last longer than water.
Soaps like Dawn and Ivory work good. They are supposedly envirnomentally safe, allowed us to use them on Federal Refuges when I did seismic work.