Hi all. Recently noticed a greeny/yellow deposit on the underside of the oil filler cap on my 1999 S3. Am I right in saying that this is probably my head gasket starting to go? I took a couple of pics but I couldnt upload them unfortunately. The gunk gets worse after long journeys. If it is headgasket is it gonna be an expensive repair? Can a garage do a compression test to check it? And would I have to get the head skimmed or is that just if it blows completely? I was planning getting revo remap so il probably get the guy to do the head gasket at the same time. any help/advice welcome!!! cheers