Viscosity was measured at various temperatures (10-60 degrees C), of a series of oligomer fractions constituted by a perfluoro-poly(oxyethylene-ran-oxymethylene) chain with two -CH2OH end units (FOMBLIN ZDOL(R), Ausimont S.p.A.), with molecular weights ranging from 400 to 9050. An unusual non-monotonic trend of the eta-M relation, which cannot be described by the typical Debye-Bueche equation eta = KM below M-c, has been observed at every temperature. An empirical equation eta = eta(M,T) is simply derived, which provides a good quantitative description of the experimental results. From an end copolymeric view, this results in a simple composition equation, in which the logarithm of the ratio eta/M is isothermally additive with regard to the weight fraction of end units of the members of the the homologous series.