This is more common than you might think. A weather change from warm, moist weather to cold weather or repeated frost (condensation) on the vehicle and frequent high dew points can create moisture to form in the crankcase. This can appear as condensation in the oil system and under the valve covers. Most of the time vehicles are driven long and far enough so that this condensation is burned off thanks to the heat of the engine. However a vehicle that is not driven very far and does not reach full running temperature for very long may not purge out the moisture. Vehicles that are not driven frequently and sit outside can also acquire moisture in the oil system. When these vehicles are driven, the engine generates some heat during the short drive, then cools. The trapped moisture condensates on the coolest part of the engine, the valve cover and oil cap. Repeated short trips will leave behind more and more moisture on these cooler parts