WVO has to be filtered down to 5 microns otherwise it will damage your engine. however, using VO of any kind can damage your engine if its viscocity is not modified in some way. Many people use a VO and petro-diesel blend, some use a VO and kerosene blend, some make Biodiesel, and others use a heated fuel tank in order to lower the viscocity and reduce the amount of deposits left in the engine.
Low pressure InDirect Injection injection systems are far more forgiving of the viscosity variances in veg oil. I'm not surprised the Merc started and ran. The lowered fuel economy noticed is in part due to the relatively inert glycerin portion of the veg oil tri-gliceride molecules.
Higher pressure direct injection fuel injection systems (VW's TDI) are less forgiving of variances in viscosity from the petrodiesel on which they were designed to run. The interim unit injector technology (VW called theirs Pumpe-Duse), was far less tolerant. The current technology of common rail injection systems from Mercedes and 2009 VW diesels and others are even less tolerant yet. Operating pressures had been climbing dramatically, at the same time clearances and machining tolerances have been reduced, all in the name of lower emissions and higher power.
The practice of heating the oil to reduce its viscosity works towards reducing one issue, but increases another. The injector pumps are cooled by the fuel coursing through them. Operating the pumps at sustained temperatures of 180F due to heated fuel is not within their original design criteria. Seals degrade more quickly, the chemical reactions (rusting, deposit accretion, polymerization) happen more readily and more quickly at elevated temperatures.
What no-one has mentioned yet, and my biggest concern with veg oil as fuel in compression ignition engines, is the water content and the dissolved mineral content, especially with re-claimed cooking oils. The ocean can be filtered to sub-micron levels, but it isn't acceptable to run it in an engine.
Conversion of the oil into biodiesel by means of chemically separating the glycerin backbone (and any salts) from the esters and allowing sufficient time for stratification and settling makes the ester fuel portion C19H40 nearly chemically identical to petroleum diesel C18H38. This 19 carbon chain length biodiesel molecule has far less viscosity difference from the 18 carbon diesel fuel molecule and significantly reduces the risk of fuel system damage.
Biodiesel is easy to make from veg oil. Unfortunately it is also as easy to make a bad batch.
ANyone using diesel secret should do a search on TDIclub or other diesel websites. Not too many good things said. I remember the water injection thread on infopop. Still don't know what ever came of it, as far as reducing problems when using WVO.