the very fact that she still is alive to tell the story negates the safety issue (i imagine the concerns are trumpeted for decades toward her, not new thing)

she's probably healthier than those safety-conscious people who buy groceries at wal-mart

by volume, plants are consist of carbon, oxygen, and nitrogen, phospate, potassium; thus major elements. they do take CO2 from air pollution to grow. motor oil, if it ever breaks down, probably will just supply C and O. lead is not even a trace element, average plants probably don't take Pb and if they do, probably extremely minute, like 0.0000001% (# pulled out of my ass)

maybe just washing it with baking soda and rinsing it with running water is sufficient, probably overkill -- don't worry much about plants, they are normally safe, that's why you rarely read news about people got killed by eating plants.

i'm starting urban gardening too (i played with aquascape before) ... it will be interesting