Lip seals made of PTFE compound are used due to their high thermal and chemical stability for the sealing of shaft interfaces in housings. For a better dynamic leak-tightness hydrodynamic sealing aids are manufactured on the sealing lip. Thus a PTFE lip seal is capable of back-pumping fluid from the air to the fluid side. The pumping rate serves as an important parameter for the dynamic leaktightness of seals with a unidirectional sealing aid design. The correlation of pumping rate and dynamic leak-tightness for bi-directional sealing aid designs is deficient. A new hydrodynamic parameter is introduced to assess the dynamic leak-tightness of a sealing aid design. The so called pressure drag is the force resulting from the integral of the hydrodynamic pressure over the surface of the sealing aid. A hypothesis is proposed, stating that in order to guarantee a dynamically leak-tight shaft seal, two conditions should be satisfied. First, the pumping rate should be greater than zero. Second, the axial pressure drag should be directed entirely towards the fluid side. The hypothesis is verified on different types of unidirectional and bi-directional sealing aid design. In conclusion the axial pressure drag is shown to be a suitable performance parameter for the sealing aid design.