Some late model Subarus and all earlier models have a Crankcase Ventilation System which does not use a PCV valve.
The only regular check necessary is an inspection of the PCV hose(s), which run from the crankcase to the air cleaner. These should be checked every 12 months/12000 miles (20,000 km) on 1970-73 models, or every 24 months/24000 miles (39,000 km) on later models. Be sure that all of the connections are tight and that none of the hoses are pinched, clogged, or cracked: replace as required.
On ff-1 models, the oil separator should be cleaned every 12 months/12000 miles (20,000 km), whichever occurs first.