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Re:  used motor oil

Part of getting "used" means picking up some rather nasty stuff both as combustion byproducts and from engine wear, including heavy metals.  It's stuff that typically goes out the exhaust pipe, but some doesn't make it that far and instead gets collected and concentrated over the useful life of the oil.

I'd guess it's safer than undiluted mercury, but that's not a very high hurdle in my opinion.

(Heck, I used long-life synthetics just so I can deal with the stuff less often.)