If he has that much in his tank he should be near full. I would say he can run it down to 1/2 empty and top it off with oil.

I am at the beginning of my biodiesel experiments. I took 3 jars and put the following in each using diesel and filtered (4 layers of cheese cloth) WVO.

1) straight WVO

2) 1/4 WVO:3/4 diesel

3) 1/2 WVO: 1/2 diesel

I boiled the WVO to get some of the water out. The diesel and oil seemed to mix homogeneously. I let them sit for a day and all was well. I then put all 3 into the fridge and brought them down to about 38 degrees F. The 2 mixes with WVO:diesel seemed to cloud slightly while the straight WVO was clear. They all seemed viscous enough to run in a car without any problems. With the weather down there he should have no problem whatsoever with running a 50:50 mix of WELL FILTERED WVO and diesel.

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