yes, confused.
WVO= waste vegetable oil, SVO=surplus vegetable oil, VO=vegetable oil.
Biodiesel starts as one of any of these (or in some cases, and not particularly desirable, animal fats or hydrogenated vegetable oils).
The problem with raw or used vegetable oil is that it is simply too viscous (thick) to burn completely in a diesel engine, most agree. This includes the legendary 5-cylinder Mercedes-Benz engine from the 300 series from 1979-1985. One can convert their vehicle with either a single tank system or a two-tank system, ranging anywhere from free (i.e. home made) to upwards of $2500 not including installation costs. EXTREMELY heated debate ensues when you get single-tank proponents in the same discussion with two-tank proponents. Both systems heat the WVO/SVO either in the tank or in the fuel lines (or both) on their way to the pre-combustion chamber, as well as filter the oil much better than the OEM fuel filters are able to. Single tank systems are supposed to enable the vehicle to start up and shut down without having to use "dino-diesel" or biodiesel. Two-tank systems are supposed to be safer for the vehicle, and do require start-ups and shut-down on thinner fuels.
In any event, if you do not achieve complete combustion, the result will be carbon deposits on the injector tips, and on the lands and grooves of the cylinders. This carbon is actually harder than the steel it forms on and alledgedly, such deposits will pretty quickly score the inside of the cylinder chambers, resulting in a gradual loss of compression and eventual death of the engine. Also, injection pumps can fail (catastrophically) if they are pumping a fluid significantly more viscous than what they were designed for. MB injection pumps are unstoppable, hence their extremely high desirability among the WVO crowd.
None of these problems exist for biodiesel, as the transesterification process thins out the fuel. The addition of caustic soda/ lye (either potassium or sodium form) and methanol (or less commonly ethanol) will pull out the glycerin which is the principal constituent in vegetable oil that makes it so thick at room temperature. It isa fairly simple chemical process, though not particularly safe (but safe enough for people who respect the dangers and take due precautions).
The only problem with biodiesel is the more rapid degradation of fuel lines resulting from the solvent properties of the methanol residue in the biodiesel (parts per million). Newer vehicles were equipped with lines and hoses made with flourinated rubbers (i.e. Viton). Most older cars have had these lines replaced over the years anyhow, but some still remain. They're cheap to replace, so this is hardly a limitation. Nobody known to the biodiesel community at large has reported problems with gasket or o-ring failures from biodiesel use.
I hope this helps...