I don't believe that it costs anything extra to use bio diesel, other than the fact that bio diesel is more expensive than regular diesel.

The conversion kit that was spoken of here, I believe, is in reference to using vegetable oil, or fryolator oil. This requires a conversion, as the oil must be heated before it is suitable to burn. The vehicle must be started on regular or bio-diesel, and then switched to vegetable or fryolator oil. It must then be switched back before being shut down, so that it can re-start.

Bio diesel is primarily a soy bean based fuel, and is very limitted in availability, and is generally more expensive than regular diesel. It does lubricate injection pumps better, and has some other benefits.

I don't think discrimination laws apply here, because nowhere in the equal employment opportunity act does it say that you can not discriminate due to "fuel useage." Probably wouldn't hurt to check with a lawyer.