A lot of oil is only superficially impressive (5)

I believe the answer is:

slick

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'a lot' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for slick that I've seen before include "Smart" , "Showing Polish" , "Bedeck" , "Film of spilled oil" , "Unwanted floater" .)

I've seen this clue in The Daily Express.

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