Q) Ive spotted a yellow, custard-like substance under the oil cap of my Rover 75. What might have caused this, and is it something to worry about?
A) The substance is an emulsion caused by water getting into the oil system. If you dont use your car regularly or do only short journeys there is a chance that its caused by condensation.
More likely, though, your Rover has suffered a blown head gasket. This has probably allowed coolant from the cooling system into the engines oil system. Wed advise getting the car looked over as soon as possible, because water in the oil can cause serious problems, regardless of how it got there.