it's an expression, meaning muscle. " It might take some elbow grease to lift it."

"Elbow Grease"-- Work harder then normal.

"Head Honcho"-- The person in charge.

I used so much elbow grease when I tighten the bolt the wrench slipped off it.

That is not idiomatic. The idiom is "elbow grease" which means "exertion" comparing it to a bottled product.
"You can get that brass to shine if you use more elbow grease." means "Polish harder and the brass will shine."

The car that the person is building will be in their garage in order to give them elbow grease you have to click on the car you wish to give elbow grease. The only exception to this rule is Police Charger that you have to complete in order to finish the Dom's Charger Challenge. I still have not figured out how to give someone elbow grease if it is in the other person's hideout.