Quite normal for this time of year. Don't worry too much about it. Just clean it out as best you can and watch the oil level for a while.
Salad cream, or mayo is caused by condestion within the engine block evaporating and trying to escape through the top of the engine. As it does, it mixes with hot oil fumes and creates this sludge. As I said, it is common this time of year for it to happen because of the temperatures differentials.
A good run will often help reduce it, whereas shopping trips will make it worse.
