Building a work truck.
 

I am starting work on a new work truck for myself. I pickep up a Subaru Brat from the late 70's I plan to start modding in the next few weeks. My Question is I have the ability to get moderate ammounts of "contaminated" diesel fuel and/or gas and waste lubricants (atf and engine oil). My goal is to tune the engine to run on either a 70-30ish mix of gas to diesel, or maybe an 80-20ish mix gas to waste lubes. The percents are just a ballpark at the moment.
Does anyone have any exp. on this? I have played with it some before with my current work truck and have seen good improvements, but its pcm couldnt adjust enough I think. Im looking to maintain the power/driveability of the vehicle as much as possible, but to also reduce the ammount of fuel it needs to run X distance.