Mad Man Mooney, used car salesman, oleaginous in spirit and person

Oleaginous

Oily in nature, either literally, or as an obsequious or manipulative personality

Example:

This is English at its very best. Easing is not one of the great events of life; it does not call for Beethoven; it is not an idea to get drunk on, to wallow in, to engage in multiple oleaginous syllabification until it becomes a pompous ass of a word like “facilitate.”
— Russel Baker, So This is Depravity (1981)

On the wooden desk above this coop sits a grinning young butcher, showing off the flesh and organs of recently chopped-up chicken, still oleaginous with a coating of dark blood.
— Aravind Adiga, The White Tiger (2008)

Disraeli once described the manner of Bishop Wilberforce as “unctuous, oleaginous, saponaceous.” And the good prelate was ever afterward known as Soapy Sam. For every man there is something in the vocabulary that would stick to him like a second skin. His enemies have only to find it.
— Ambrose Bierce, The Devil’s Dictionary (1906)

Etymology:

Olea ultimately comes from the same Latin word as both “olive” and “oil”.