The most widely used method of measuring bearing oil film thickness (BOFT) is the “capacitance technique”. It has two implicit assumptions that need to be carefully considered. These are: (1) The bearing surfaces are smooth down to the molecular level. (2) The dielectric constant is uniform across the interface. This paper examines each of these assumptions and finds: (1) The real MOFT is smaller for a rough surface than the reported MOFT (2) The real MOFT is larger if the VI improver concentrates at the bearing surface than the reported MOFT (3) The magnitude of the first effect is larger than the second. For values of MOFT greater than one micron, the reported MOFT values are sufficiently independent of surface roughness and possible adsorption effects that the standard MOFT equation can be used.