[sammyg2]

Just to clarify: refining the oil means to strip out all the stuff that isn't pure oil molecules of a specific type and size.

They take the old oil and filter it first to get rid of entrained solids.
They add water and amines to strip out the sulfur.
Then it's heated and all moisture is flashed off.

Then they heat the crap out of it, prolly close to 500 degrees or more and pump it into a very tall vessel with different trays in it at different levels.
As the liquid oil goes into the tower somewhere in the middle the pressure drops and the ligher molecules go through a phase change and vaporize, basically boil off. As that happens they rise to the top of the tower and are collected and drawn off.
The really heavy stuff falls to the bottom and it's pumped out. Somewhere near the bottom but not all the way down is a tray where they make the ideal cut.
In other words that's where the oil they want barely goes back to a liquid state.
The internals and temperature/pressure controls are much more complicated than that but it's distillation and if done correctly, they can maximize the good molecules and minimize the bad molecules.

Refining used motor oil if done correctly can make very pure oil with little or no bad stuff. In organic chemistry there is ALWAYS a mixture. You just maximize the good and minimize the bad.

Look at it this way, would you use crude oil to lubricate your motor? Of course not. It wouldn't make it to the end of the driveway.
But we can still get good motor oil out of crude through distillation.

But if they start with stuff that's already motor oil it's actually simpler to seperate all the different ranges of molecules because you're starting with something that's already near pure. It's just too costly to make sense, even if uncle Sam makes us all pay for it.

The real question is this: what do they do with all the toxic waste? The sulfur can be purified and reclaimed, but there are heavy metals and other bad things in used motor oil.
Refining it makes that bad stuff that's left over more concentrated. Maybe they can incinerate it but that's be almost impossible in Kalifornia.
Most likely they have to pay to send it to a toxic waste dump to the tune of $800 per bbl.