When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
You have to burn the organics out of used cooking oil and you have to remove any water/moisture. That is why processing WVO is a somewhat complicated process. As wulfman stated, you need to heat the WVO if the temperature lowers enough to where the oil starts solidifying or gelling. About 4 years ago I bought a processor to take WVO and clean/remove moisture to run in my 6.0. It was a lot of work for the small gain. When I got fairly clean used oil it worked well, when someone also dumped other liquids into the oil containers, I never knew how things would turn out. I ended up selling the equipment and happily buy my Diesel, either petroleum or Bio, from a commercial processor.
In addition to the comments above, WVO can get into the crankcase (blow by the piston rings) and react with the oil to make your oil into crisco. I'm sure it would take more than 2 gals of WVO -the main point really is WVO and biodiesel are very different and there are a number of additional considerations with WVO.