In my experience (we have two VW diesels, and two early 1980s Mecedes
diesel) is:

1) In a VW engine, straight (or anything more than 10 or 20 percent)
veggie oil causes coking/carbon build up. Not recommended.
2) In a Mercedes engines, there still is coking and carbon build-up, but
it's not as bad as a VW engine.

My recommendation: don't use Veggie Oil. Use/Make biodiesel.

Both the VWs and the Mercedes' engines are much cleaner after using
Biodiesel.

And, I do experience a little less mileage with veggie oil or Biodiesel.
There is simply less energy in a gallon.

Jon

> On the straight "previously used" vegetable oil, the oil was filtered,
> but otherwise not tampered with. The Mercedes diesel got 30 mpg, not 33
> mpg. The 33mpg was on regular diesel fuel. Even so, that is pretty
> impressive. In a pinch, you could go into the grocery store and buy
> enough vegetable oil to get to where you wanted to go.