Hello, have had only cold air blowing through vents for few weeks, last week checked coolant levels and found greyinsh gulk in resevoir. Told by a friend that greyish gunk isnt abnormal in this weather using car for fequent short journeys. Tonight car began to overheat on motorway, lifted bonnet later on to find coolant/oily gunk splattered all over underneath coolant resevoir and coolant low. Checked under oil cap and no gunk at all. Is this a Head gasket failure or could it be something else? Any advice greatly appreciated as I'm going to local garage in morning 👍