I combined used oils in fuel systems on Cat dozers and scrapers, other machines too for several years in the eighties. It was an accepted practice then before emissions and the environment became such an issue. We filtered the oil carefully, also made sure that no one dumped cleaning solvent or coolant into the collection tank. Most of the engine manufacturers had specific standards for what percentage of used oil to fuel you could run. I would agree with some here that older machines up to about 1990 could run safely with used oil, after that, with the changes in fuel system technology, I don't believe I would try it.
One project office manager that I worked with in Pennsylvania bought old home heating oil without my knowledge for use as fuel in our machines. He had done this on his previous project and saved a bundle on fuel costs. However, the high acid level of that fuel etched the injectors and the high pressure injector pump in a newer Cat 245 excavator and ended up costing us over 25000 bucks for new parts and labor. Thinking back, I believe it cost us an engine in a 777B haul truck too, which was not cheap to replace and there was a week or more down time on both machines to consider.
I burned waste oil in a burner that heated our shop in Colorado. Believe we put about 50000 gallons through it in about two years. The heating element gave us constant trouble and the fuel filters were also in need of replacement nearly every week. The ash built up inside the heater tubes till they plugged up and when we used a shop vac to suck the ash out of those tubes, it blew right out of the vacuum all over the shop. It was an exceptionally fine powder, like talc. Had to dismantle the furnace and wash the tubes out after that fiasco. But that furnace kept the entire shop warm without the use of the infrared gas heaters located in rows on each side of the shop. I don't think you can use waste oil burners in Colorado anymore, not legally anyway.
At the time (1992), Komatsu was injecting small amounts of engine oil into the intake manifold on WA600 loaders, of which I had two. This resulted in the plugging off of the air compressor discharge tube with a brownish coke like material. This happened to both machines at about the same time with maybe 3500 hours on each machine. After replacing the parts and realizing what caused the problem, we plugged off the line feeding the engine oil into the intake. Just an unintended consequence of what some engineer thought was a good idea.
I do not know how Cat and others are burning used oil in their machines today other than the method does not use oil collected from several sources. The oil Cat uses is from the individual machines own engine, which as I recall, was Cat's recommendation years ago, which we ignored in our zeal to get rid of waste oil as cheaply as possible.
Back then, we were under the hammer to be very careful about allowing waste oil off the projects, as the company had just gotten sucked into a federal cleanup at a used oil refinery in Salt Lake City, for which they were paying a large sum of money. As I recall, only one 4500 gallon load of company oil had been delivered to the refinery, but the company ended up being one of the top five firms cited and having to pay. We thought it was mostly the result of the outfit having deep pockets if you know what I mean.
We didn't let any waste oil go off the job in Colorado, but in other places we did. In that time I sold waste oil for as much as 45 cents a gallon (in Maryland), and paid as much as 25 cents a gallon to have it removed in eastern Pennsylvania. I also started checking the places our waste oil went pretty closely as that was a time when some pretty serious problems were discovered with waste oil haulers and collection or refinery facilities. Indeed, there was a used oil barrel reconditioning facility in Grand Junction that was forced to close because of pollution problems. They'd been in business for decades and the ground the plant stood upon was heavily saturated with all kinds of bad stuff, and they were right on the edge of the Colorado River.
There is a lot of soot and acid even in today's used motor oils. Take that into consideration if you decide to burn the oil in your engines. I don't know that it's worth the trouble really, the potential for expensive repairs and downtime are very real, plus the added expense of extra fuel filters or pre-filter fuel processing equipment. New oil isn't cheap either, which the engines that burn their own oil use in varying amounts. Next consider the effect the extra ash and soot have on catalytic converters and exhaust systems if waste oil is burned in systems with which equipped. User beware!