You can get 'white gunk' with an ok head gasket. Lots of short trips that don't warm the engine up enough to drive off the water. Generally not so at this time of year, but I bet it has occurred.
Oil in the coolant tends to hang around, and you don't need much to see it - and it floats. If you ever do need to replace a head gasket cos water is getting into the oil or visa versa, you do need to quite thoroughly clean the cooling system otherwise you'll be worried for ages that the new gasket is leaking too.
