What is the gunk collected around the oil filler cap in my Porsche 996 / 997 / 991?
A yellowish-emulsion under the oil filler cap in a water-cooled 911 may look bad, but it is usually nothing to worry about.
When used for short journeys, particularly in cold weather, there isn’t always time for oil vapour and condensation to be recycled into the block and sump, and the result is a residue under the oil filler cap. Typically, this can just be wiped off with kitchen paper.
However, you should also check that there is no sign of the emulsion in the oil itself or in the coolant tank. That could be an indication of head gasket failure, so you should book an inspection by a Porsche specialist to assess the extent of the problem.