Lifters most likely, you may get away with just cleaning them, that is if you don't mind removing the top half of the head, i.e. cam cover, camshafts etc.

Or it may well go by itself given time, someones giving it a bootful can help as this gets the oil hotter and thinner, and so may well clear the blockage, this is assuming you have a good quality oil and it's been changed regularly etc.

Martin