In this work, we adress the issue of sealing performance of metal gaskets using a deterministic approach that allows the analysis of creeping viscous flow and diffusion through a tight contact between rough surfaces taking into account surface deformation. Our analysis is focused on rough surfaces exhibiting fractal properties, and our purpose is to study the validity of the use of synthetic fractal surfaces as a representation of real ones. Two kinds of real surfaces, obtained from two machining processes - lapping and sand-blasting - are considered. After checking the fractal nature of these surfaces, equivalent fractal ones are synthesized. Distributions of contact areas on the one hand, and transport properties K (for viscous flow) and D (for diffusion) on the other, obtained from real and synthetic surfaces are compared for a wide range of tightening. This comparison leads to the conclusion that the fractal representation is adequate to predict mechanical and transport properties of a contact between lapped or sand-blasted surfaces. Finally, using synthetic surfaces, it is shown that sealing performance of a rough contact decreases when the arithmetic roughness Ra and the fractal dimension Df increase.