Yellow oil sludge on oil cap?

Yellow or "milky" sludge on oil filler cap is caused by short, frequent driving and not allowing engine to reach operating temperature because crankcase vapor, inside engine, isn't being allowed to "boil" off of crankcase and starts to film on engine block. Shutting off engine when it's not at operating temperature will cause it to stick to engine then restarting a cold engine, driving short time will start this all over again. Always allow engine to reach operating temp, it's ok sometimes but not all the time.