Mixing waste motor oil with traditional diesel fuel grades necessitates a processing and conversion technique referred to as "black diesel" production. Some people have their own variation to the process and experiment with their own methods for filtering out the sediment and impurities from used motor oil. As a word of caution, when mixing waste motor oil, even after the filtration processes have occurred, there is still a risk of damaging or causing permanent failure to your fuel system for your diesel engine, including your fuel filter and injectors.
Don your safety equipment. For much of the process, you will be heating the diesel oil to extreme temperatures that could cause serious burns if the motor oil is accidentally spilt onto your skin.
Use a funnel to pour the waste motor oil into a dedicated pot or flask set apart for the "black diesel" production process. Heat the waste motor oil to at least 148 degrees C on a hob, using a cooking thermometer attached to the pot to measure the temperature. This will thin out the oil so that you can filter out its impurities. As the motor oil is heating pass a magnet through the oil to pick up some of the larger metallic pieces that may have been dislodged into the motor oil.
Set up another pot and funnel system to catch the oil as it passes through a filter. While it is still heated, pour the waste motor oil through a 20-micron filter at least twice in order to begin the filtration process for removing sediment and impurities. Continue this same method using progressively smaller filters, moving down to 15 and 10 microns, reheating the motor oil, when necessary.
Pour the waste motor oil into a centrifuge, which can act as a more refined filter. Make sure that the centrifuge you are using is able to withstand extreme temperatures for the motor oil, which will yield the best result when it is heated. Continue to use the centrifuge, passing all of the motor oil through it multiple times until the sediment that you are filtering out is not showing up on the other side of the filter.
Pour the waste motor oil into a flask for distillation. Heat the motor oil until it begins to condense and evaporate into the elevated flask, which is part of the distillation system. Take the leftover motor oil sludge in the primary distilling flask to an auto parts store for disposal. The motor oil remains in the second elevated flask is ready to be mixed with your normal diesel fuel.
Pour the "black diesel" into a fuel can and pour small amounts, at first, into a nearly full fuel tank. Slowly increase the amount of "black diesel" fuel that you are inserting into your fuel tank and engine. Watch for signs of decreased power, which are inevitable. However, if you begin to hear regular misfires in your engine, then you may want to discontinue the use of your "black diesel" or reduce the amount of "black diesel" in your diesel fuel mixture.
Pour fuel additives into your diesel engine's fuel tank to clean the fuel injectors. Inspect the fuel filter on a regular basis while using black diesel for clogging and replacement.