Might suggest the seal on the oil filler cap needs replacing, or a split oil breather pipe. Or if your running a pancake filter and have one of the tiny pancake filters on the oil breather then you'll get the white gunk all the time and should fit a proper air filter and breather pipe.
I had similar on my old type4 engine. i put it down to condensation like the others mentioned. i was doing alot of short runs where the engine wasnt really getting up to temperature and found some inside the rocker covers
Short runs or too heavy a weight oil, like 20/50, will do it. You need to get the motor to proper operating temperature to vaporise the moisture and the air-cleaner should take care of the rest.