I used a lot of waste diesel engine oil as a fuel back in the late 80's and early 90's in mostly Caterpillar and some Komatsu and Hough equipment. I didn't have any trouble with it as we used about a 15-20% ratio of waste oil to #2 diesel fuel mixture. It did plug up fuel filters a bit faster, though we never had any real problems with that as they were changed on a regular basis. We didn't note any trouble in our engine oil samples either.
Most of the engines that used this were Cat D346 pre-cup models. The rest were more modern 3400 series direct injection Cats, and the Komatsu's and Hough's were direct injection models too. I did this on three separate projects. The last job, we also used waste oil to fuel our shop heater. We only used waste diesel oil in the engine fuel blending, but the heater tank got everything. The heater worked well the first two winters (Western Slope, Colorado), but began to give us trouble after that. We discovered that the ash buildup in the burner tubes was considerably more than expected. The fuel heater also started to give us trouble by not working or barely working. None the less, we got rid of several thousand gallons of waste oil and did not have to pay to get rid of it. fix is correct, do NOT pump straight off the bottom or bottom end of the heater oil tank. The water, bacteria, and antifreeze will do your end use no good. The slush must be drained off every so often.
Now, that all said, one must also be careful just what kind of fuel goes into your machines fuel tanks. Do NOT used old Kerosene or home heating oil that has been retrieved from homeowners private oil tanks. It will most likely contain a lot of Sulfur, water, bacteria, and acids. I received a newer Cat 235 excavator off another company project in Pennsylvania where the office manager had been in charge of purchasing fuel. Ever zealous to cut costs, he bought several loads of old home heating oil as fuel for that projects equipment. The 235 engine had one of the first very high pressure fuel injection pumps, and the machine gave us trouble from the very first day on the job. I finally had the Cat House sent a tech down to work on it and he found that the injection pump and nozzles were faulty.
In their fuel shop, they discovered, upon disassembly, that all the internal parts of the pump and injectors were etched severely from the acid, so badly in fact, that the entire pump had to be thrown out and replaced with a new one -- at about $30,000. There were another 3000 bucks in the service fees, etc. Then we also had to remove the fuel tank and have it cleaned out. There was about two and a half weeks of down time as well.
Waste diesel engine oil can be blended into your fuel, but it has to be tightly controlled and managed. With regard to today's engines, I don't believe I would take the chance with it, especially with OTR fuel in trucks, etc. Even using it for heating purposes poses a big problem with environmental laws being what they are in most places.