Why did my air filter fill up with engine oil?

I just
is the breather clogged?

maybe the motor is tired and experiencing blow-by?
thoroughly cleaned out the breather with gasoline, then washed it in detergent and dried it with a hair dryer. I then filled it with 30W motor oil and drained the excess oil, as the FSM recommended. The engine has good compression on all six cylinders. Very puzzling.