As newmineral oil is most definitely not pure (generally contamination class 6...7 according to NAS 1638), it should be filtered on filling the tank. Filler filters and breather filters comprise an air filter to filter the air flowing into the tank and a filler sieve to retain the coarse dirt during filling.
It is appropriate to combine the filler filter with the flushing power pack, as otherwise the filling process will take too long with cold oil.