Well, FWIW, I've been burning a mix of 5% WMO, 15% Biodiesel, 80% #2 diesel for several thousand miles now. At 5% it doesn't even make any extra smoke.

Oil that gets past the rings is a liquid layer on the cylinder walls and therefore doesn't combust very well or very completely. That is why it makes smoke - the smoke is oil that didn't fully combust. Small quantities of oil mixed with fuel and sprayed through your injectors is atomized well enough to burn cleanly and doesn't cause excess smoke. Hell, diesel is just super thin motor oil to begin with.

The WMO I burn has been filtered to 1 micron, and it is only MY oil that has come from MY vehicles, so I know it doesn't have any solvents or other nasties in it. I figure I've already paid $12 a gallon for it so I might as well get the benefit of the BTUs out of burning it once it isn't suitable for lubrication anymore. Rather than give it to someone else so they can make a buck off of it.

I am in charge of what happens to the used oil from our state-wide fleet of vehicles where I work. I visit the oil recycler that our used oil goes to for an inspection every other year. I can tell you 100% for sure that a large portion of the oil that goes to recyclers gets filtered and treated and is then sold as furnace fuel. Most of the rest of it gets further refined and has its addatives replentished and then becomes recycled oil that can be re-sold and used again as engine oil.

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown