Any vegetable oil will make an adequate form release. Some common oils are soybean, corn and sunflower. Cost is a factor, since you'll need a lot of it for a large construction project. The oil doesn't have to be food grade. Talk to a local rendering company and obtain old deep-fryer oil pumped out of restaurant holding tanks. Spray or brush the oil onto the concrete contact surface of the forms and let soak in before using the forms.
Sidney Freedman, director of the Prestressed Concrete Institute in Chicago, states that carnuba wax is a suitable form release agent. Carnuba is commonly used as furniture or shoe polish, and the wax is in paste form. To spread it on molds, use a stiff brush. Leave the forms in the hot sun, which will melt and soften the wax so it penetrates the wood. Obtain large quantities of wax from a chemical supply house.
You can make an oil/wax mix compound. Heat the wax so it becomes liquid. This should be done outdoors using safety precautions. Dissolve the wax into warm oil and spread it generously on the forms. Let the mixture soak in before using the forms. It may take two or three coats for the mix to fully penetrate wooden forms.
In layering, apply the warm liquid wax first to the forms, let it soak in, then spray or brush on oil. The wax first seals the form wood against water penetration. The oil lays on top of the wax to forms a second barrier that prevents the concrete from sticking.
Freedman notes that lard and tallow make an adequate form release compound. Non-food-grade lard is available from rendering companies. Straight lard should be warmed up to liquify it. Freedman warns that lard turns rancid, so the forms should be used immediately after application.