Earlier today I tried an old unopened bottle of Dokumol mid print session as I wanted something stronger working than PQ Universal for ILFORD Galerie grade 3. However the Tetenal developer (at least 5 years old) produced no or virtually no image on the fibre base paper after 3 minutes; I've not seen this before but then I've been surprised by how well older stock often works (not Dokumol). I can't find any product information / pdf from Tetenal print developers, but I know they publish documentation for film developers.
Tom
Tom