[RWebb]

Used engine oil is a toxic waste product and usually is picked and recycled by a company with special certifications, like Safety-Kleen.

It contains a nice mixture of bad things, including combustion byproducts, and various metals. It is a known carcinogen, which is why someone who deals with it often uses gloves to avid skin contact. I should too but I don't - based on age and a long history of not doing so before anybody figured out the toxicity. So, for a DIYer, I'd say up to you.

Burning is allowed in specially designed incinerators. My guess is they send the ash (heavy metals & etc.) to special toxic waste landfills, tho it would be nice if they could recover & re-smelt it.

Some yokels burn it illegally, polluting the area around their cabins.

In contrast, used gear oil stinks like crazy (mercaptans and other sulphur compounds) but does not have any combustion byproducts (unless you have a really hot shift hand).

Fun Fact: EPA should have regulated used engine oil as a hazardous waste (CERCLA is the law) but decided not to due to it being used everywhere.

New engine oil is not particularly toxic at all, less so than crude oil, but is a poor substitute for an organic olive oil in cooking.