The degradation of heat transfer on the shell side of this exchanger will depend largely on the type of contamination to which you refer. If the lubricating oil has built-up as a liquid in the base of the exchanger, then the loss can be estimated by the volume of oil present and how much area may be occluded by the liquid. If the hydrogen is at elevated temperatures and could react with the oil to form a fouling material, then it may be best to estimate a thermal conductivity and thickness of the fouling material and determine performance loss this way. Try Heat Exchanger Fouling – Mitigation and Cleaning Technologies by H. Muller-Steinhagen (IChemE publication).