This case is rather complicated, so let's commence with some general statements.

Permeation through - and viscous flow along - the O-ring depend - besides the temperature and pressure of the hydrocarbonds, oil stream compounds - on the mechanical properties of the elastomer.

An elastomer with a low elasticity or Young's modulus will have a relative small pore size between O-ring and pipeline, but a high permeation rate. A ductile materials will generate large sizes, but a lower permeation rate.

Secondly, species with a low vapour pressure will condense in the pores, see also: Kelvin Equation in Wikipedia.

Condensation (also because of the probable high system pressures) will reduce the diffusion rate of the chemicals to a significant amount, also the permeation rate of the gases in the mixture.

In industrial applications, the loss by permeation usually exceeds the loss by viscous flow, because of the above described hydrocarbons condensation phenomenon and the permeability surface relative to the surface available for viscous flow.

If this is not the case, one should realize that if the free diffusion path is larger than the diameter of the pore size between ptfe elastomer and wall made from steel, the diffusion path becomes dependent on the diameter of the pore (Knudsen diffusion, see also Knudsen Number in Wikipedia). Please do not hesitate to contact us for more assistance.
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