How can I reduce Crystallization temperature of seed oil to be used in winter as biodiesel?
Waxing and pour point are typical specs for fuel. How cold is your winter? If it is very cold, and your tank is above ground, there is little to be done. Insulate the tank (metal-backed rock wool) and heat it.  Use a more unsaturated feedstock with no saturated components. That lowers its pour point and removes waxing.  Did you dry your fuel?  Unsaturated fuels oxidize and deposit varnish.  "Biofuel" is a good substrate for microbial growth.

Enviro-whinerism, sustainability, social consciousness, are Luddite lies.  They assume thermodynamics is amenable to legislation and good feelings, and weather is always mild and cooperative.  None of that obtains in the real world.

http://en.wikipedia.org/wiki/Pour_point 
http://www.infineum.com/Pages/pourpoint.aspx 
http://www.aftonchemical.com/Solutions/LubricantComponents/Pages/PourPointDepressants.aspx 
http://www.globalp.com/products/product.cfm?productID=32 
Chemicals