Used oil for heat
 

I have some used motor oil piling up in the garage and would like to dump it into my heating oil tank if possible. Is there any risks? I've heard of burning it in deisel engines with filtering it before use. What type of filtering is used? A pump fed system or just pouring it through a coffee filter?

Should I thin the motor oil with some kerosene or alcohol? It should make filtering easier, but I'm only adding 5 gals of oil to a 250+ gal tank.

You need a waste oil burner to burn untreated motor oil (they can also burn untreated SVO/WVO). Your mechanic might heat his garage with a waste oil burner. There are instructions for constructing a DIY waste oil burner:
https://www.motherearthnews.com/Do-It...il-Heater.aspx
It sounds like a lot of work and possible danger, but it would be an interesting project.

I suspect that diesel engines are mostly more robust than heating oil burners. I'd guess that they can handle the garbage found in used motor oil better as long as particles are filtered out, but I'm pretty sure that it would require a better filter than a coffee filter. I know that WVO users use better filters.

Thinning it with kerosene should be about the same as just dumping it in the heating oil tank; kerosene is approximately the same stuff as common home heating oil. If you're going to the trouble of mixing stuff, I wonder if a process similar to making biodiesel can produce a similar result from used motor oil?

I find if it's been standing for months all the crap settles to the bottom, and the top is relatively clear. A couple of times when I've been flat broke and Marvin needed oil, I topped him up with a quart off the top off a "well settled" jug. Usually only when I was approaching an oil change anyway though. So, if you carefully pour off the top 3/4 of all the stuff that's been sitting for more than a couple of months, you might find that it doesn't need filtering. The stuff that's left is gonna be thick black crap, I'd try and find a big metal container for it, then light a fire under it, get it nice and hot and pour it through coffee filters as a first stage and see what it looks like after.

Don't think you'd get far with a biodiesel type process, probably need to steam crack it or something.