Consumer reports wished to see the difference between operating a Jetta TDI on diesel, biodiesel, 5% biodiesel, and pure cooking oil.
Cooking oil was found to have the lowest emissions out of all four fuels tested, with an unnoticeable difference in performance. Here is the chart that they produced:
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For a filtration system that simplifies fuel collection, see an Oil Centrifuge, and for a fuel system which switches over to vegetable oil and finishes the purge on shutdown automatically, and lets you know if there is water in oil, among other enhanced features, check out the Diesel Conversion Kit from OM!