If you've had the `Oil Sensor Workshop!` message flash up on your dash there could be a number of faults. If you've checked the oil level, and or replaced the sump sensor then it could be that the bonnet micro switch is faulty. Below are the part numbers for the bonnet micro switch and the clip that holds it.
Golf MK4 Bonnet Micro Switch Part Number: 1J0 953 236 B (1J0953236B)
Golf MKIV Bonnet Micro Switch clip part number: 3B0 953 248 (3B0953248)