I had the same question not long ago:
http://www.apug.org/forums/forum37/1...aded-what.html
Following the majority opinion of responders, I bought a new set of filters. With ~35€ now out of my pocket I found that both in appearance and function the old ones (purchased 30 yrs ago) were identical to the new ones.

+1 on the advice of pentaxuser. Make sure the other variables are known: fresh paper, fresh dev; remember that there is hardly any difference between grades 4 and 5.