I keep getting white/yellow scum on the oil cap. I have changed the oil cap as the rubber had perished but still get the scum. I took it to a garage who told me that there was nothing wrong with the head gasket and it was probably because I do not drive the car much. I do 18 miles a day so I dont think this is the case. They did say that the rocker gasket was seeping. The oil is clean and there is no scum on the dip stick. Has anyone got any suggestions to what the problem might be and any solutions?