[nextcar]

Echoing the above comments... Safety is the obvious concern.

Pretty much this is like getting up to go to the bathroom while the seatbelt sign is on. If you ask, they will say they cannot permit you to... but they (generally) will not prevent you.

I have checked many used car parts on commercial flights, often with grease residue. It has never been a problem. I always wrap in shipping film and pack them well so there is no chance of seepage/saturation of the cardboard.

I would not attempt to check flammable liquids or volatile liquids - that is just stupid. But grease/oil residue on a car part is less flammable then a lot of toiletries!