To meet the stringent demands set on future gear transmissions, and to allow for manufacturers to make them more efficient and durable, it is essential to understand what mechanisms that govern their performance. This work was launched with the chief targets of establishing key competence in gear lubrication. The fundamental film-forming mechanisms in rolling-sliding, heavily loaded, and rough surface elasto-hydrodynamic lubrication (EHL) were explored by means of ball-on-disc experiments arranged in a highly idealized setting. Tribological tests were conducted to explore the interplay between surface roughness and the transition between the EHL, mixed lubrication (ML), and boundary lubrication (BL) regimes. It was found that the onset of film breakdown is fairly well represented by the classical Λ-ratio (the film thickness over the surfaces RMS or Sq level) when surfaces are closely Gaussian. Accordingly, the criterion typically considered for full film (FF) EHL, Λ ≥ 3, was confirmed, and in addition, moderately revised to Λ ≥ 2. However, it was also found that when asperity peaks have been subjected to running-in wear, the validity of the Λ-ratio no longer holds. A reduction in the Sq parameter by approximately e.g. 15 % was found to cause a reduction in the EHL lift-off speed by a remarkable 90 %. This means that FF-EHL is possible even in cases where the Λ-ratio falsely suggests boundary lubrication (BL). To better understand this discrepancy, a set of well-controlled running-in and measurement routines, together with a novel surface topography transformation tracing technique, was developed and employed for assessment of the topographical transformations associated with running-in and EHD lift-off. Accordingly, from a range of running-in tests conducted under a wide variety of conditions, it was found that the running-in improves surfaces micro-conformity of local surface irregularities, and consequently, their hydrodynamic load carrying capacity (HLCC). More specifically, the reduction of peaks and growth of their radii were attributed to the establishment of the micro-EHL regime – a thin film state which allows for FF-EHL even when the nominal film thickness is less than the measured surface RMS level. With this in mind, and by leveraging existing well established EHL and surface metrology theory, a new film parameter was deduced, Λ∗. The parameter accounts for the fluid-structure interaction (micro-EHL) induced by essentially any type of machined surface structure, isotropic or anisotropic. Validity was made possible through the utilization of the above-mentioned test and surface analysis techniques. Furthermore, the parameter was derived to be accessible for the broader spectrum of researchers and engineers interested in a straightforward, yet accurate, assessment of the lubrication quality. The ability to accurately predict the mode of lubrication is important in the manufacturing of tribological interfaces for optimal efficiency and durability – an example was provided. Finally, the proposed model was used to assess whether a novel additive technology, P-SiSO, can be employed as a running-in agent for improved micro-EHL. Such an in-situ modification would enable for surfaces to be prepared rough in production and thus respect the restrictions imposed on the manufacturing economy, but without compromising the lubrication quality in service. The remarkable performance observed indicates that the strategy is promising indeed.