Probably just due to cold, short runs......engines produce lots of water / chemical condensate and usually this boils off (or it should in the summer). However, as the engine rarely gets scorching hot, this condensate condenses (
) at the coolest part of the engine, i.e the rocker cover. If you were to take the cover off, you'd see loads more inside!
In the good old days, this condensate vapour was allowed to breathe away outside, and then someone had the bright idea of routing it through the air cleaner, where it choked up the carb and filter
It has to be burnt nowadays to prevent pollution.
So - check the coolant regularly, wipe away the grot from the oil filler cap from time to time, and worry ye not.
P.S Diesel drivers will be smug here, as its only the petrol engines that are really affected
John