There really isn’t anything wrong with using straight VO but there is a problem with the oil if it is unrefined or straight out of the bucket – mainly with the fuel system becoming clogged with fat that is removed with refining of the oil.
I know one person who used straight peanut oil that was used once to process food, I can’t figure out what they did but he would get it for $1 a gallon in drums. Well he ran it all summer long and tried to go back to straight diesel, but had to replace filters several times to get it running right. I warned him that running straight VO will mess up things.
I get a kick out of these inventors who create these highly filtered systems that claim you can use any waste oil – wrong, still have to refine the oil after you filter it.
OH by the way Soy oil is one of the more common oil used in commericial Bio-Diesel.