Originally Posted by joshofalltrades
dieselrod, i look forward to hearing your results from centrifuging WMO, both from gas and diesel applications. my stock is primarily gas engine oil, with some ATF, diesel oil, gear oil, and possibly small amounts of whatever else i have around (brake fluid, solvent, etc), as until recently, all waste fluids went to the same place.
I have done a fair amount of centrifuging waste oil. I have an IBC in the back yard into which I dump all my waste oil (engine oil, ATF, hydraulic fluid) and let settle for a while, then I pump off the top into a centrifuge barrel.
I've done 12-15 passes on this oil (which is black as midnight) after heating to 195 F, and while the centrifuge does pull some crap out, it doesn't change the color of the oil at all. Even after mixing the finished product 50/50 with No. 2 diesel, I can't see anything through it, even with a really bright light.
The centrifuge seems to be doing what it's supposed to, but I can confirm what lots of other people have said: a pressure-operated centrifuge (mine is a PA Biodiesel 120 GPH model) does not get the carbon out of used motor oil. Maybe a direct-driven, higher-RPM centrifuge would do a better job; I don't know.
I can tell you something interesting I discovered, though. Lots of people on alternative fuels forums will tell you not to thin oil with diesel fuel, because according to them diesel fuel is not a solvent. They say you need to use kerosene or gasoline.
Well, it's not true. I took a sample of my centrifuged oil and mixed it 50/50 with No.2 diesel in a clear glass jar and let it sit for a week. There's no separation whatsoever. I also took another clear jar and put some virgin ATF and virgin 5W-20, and mixed those 50/50 with No. 2 diesel, let it sit for a week. Same result; no separation. Also, mixing 50/50 with No. 2 diesel noticeably reduced the viscosity of the oil. So I would conclude that you can use diesel fuel as a thinner without worry. It's not the cheapest solution, but it works well.