Just one word: plastic.

actually, the word unctuous came to mind today:

1a : fatty, oily
b : smooth and greasy in texture or appearance
2 : plastic <fine unctuous clay>
3 : full of unction (exagerated, assumed, or superficial earnestness of language or manner)
© 1996 Zane Publishing, Inc. and Merriam-Webster, Incorporated

the second sense here, plastic, isn't something I recall seeing in use; but here's the second sense of plastic: capable of being molded or modeled: plastic clay. that ties in nicely with unctuous[2] without becoming circular.*

but here's a more interesting thing, I thought:
plastic has developed a sense of 'artificial', having a quality suggesting mass-produced plastic goods. and here we also have unctuous, with a sense of exagerated, assumed, or superficial earnestness.

Is this all just coincidental happenstance or is there something else going on here?

*defining A in terms of B, and then B in terms of A.