Things on the numerator side that make oil film bigger are, in order that you come across them in the formula: * radius - more shaft diameter, more film thickness; * viscosity - more viscosity, more film thickness * Speed - more RPM, more film thickness. On the denominator side (make these smaller to make Sommerfeld number bigger) * radial clearance (smaller radial clearances, more film thickness) * Applied pressure (smaller loads, more film thickness). So a thin lubricant, e.g. for formula 1 use (tight clearances, very high RPM) is of zero suitability for a diesel with high BMEP, and low RPM potential. Stribeck curve uses the last portion of Sommerfeld to demonstrate hydrodynamic, mixed, and boundary regimes.
Note...more viscosity, more likely to be hydrodynamic. More speed, more likely to be hydrodynamic. Less load, more likely to be hydrodynamic. Note that designers are increasingly allowing MOFT to fall into the contact zone, and are using friction modifiers to prevent failure, and lower friction. These additives are NOT increasing the film thickness.