I think i might have a possible head gasket problem. I have noticed what looks like oil in the header tank in my corsa. It looks like thin lines of oil floating on the top of the water. I have flushed the cooling system but it is still there. I have done a compression test and all four cylinders are fine. I have checked the oil cap and the dipstick and no water has got in to the oil. Wondering if i could have a porous head or a damaged head gasket that has not yet affected the compression. Does anybody have any ideas?