If you get dipped your car will be impounded until you cough up the fine.
They would not crush your car.
The fine is a on the spot fine of about 600 pounds.
Using sump oil as fuel is illegal because there has not be any duty paid on it.
People mention about a sniffer but that a load of bollocks.
.
the sniffer is an electronic sensor that's shoved into the fuel and tests it's content
sadly (for them) vehicles do get seized AND they'll be getting a big bill for the back tax