This is how TDIClub forum member DuluthRooster starts a post explaining how he ran a Jetta Wagon on waste vegetable oil for about 50,000 miles but then, sadly, had a terribly negative experience and ended up replacing the engine. He details the damage done to his engine by using VegOil that probably had too much water in it also probably also wasn't filtered enough and the steps that he needed to take to figure out what was wrong and how to fix it over in the forums. It's great reading if you want to know the dangers of using waste oil in your car, even if you think you know what you're doing.
I don't want to steal his thunder too much (his pictures are worth checking out, too), but here's how he ends his post:
I will conclude my tale with a comment about WVO. I thought I was doing everything right but I damaged my car. Some mistakes I know I made were I didn't increase my oil change intervals until it was too late. WVO permeates your crankcase oil and does not burn off so you can have loss of lubricity. Also water may have still been in oil evidenced by the blown off piston parts.
I think it was Drivbiwire that said that any money you save by running WVO will eventually be given back. At the time I thought he was a scrooge (well he is ). Well check this out.
Over two years I have saved about $5000 running WVO. In the last three months I have replaced a rear seal ($600), a turbo twice ($1400 VNT-15 times 2), new used engine ($2000), $1800 labor total to three different people, plus me chasing my tail for three months changing filters, fuel line, pulling injectors, etc. [...]
My next project is a biodiesel reactor. From now on it is B100 or D2 for me.
We certainly like the idea of WVO here on AutoblogGreen, but we also want to make sure that if you use it your experience is a good one. Take this as a warning, then proceed cautiously.