i checked the oil level on my vauxhall astra today and it was a bit low so i went to top it up and when i removed the cap the underside of the cap was covered in something looking like mayo and so was the top part where you fill with oil i know this can generally be a sign of a head gasket fault but there are no other signs no oil in the coolant engine isnt overheating no misfiring is there anything else what couldve caused this