Scuffing is a severe form of surface damage which limits the performance of lubricated sliding machine components. It has no generally accepted criterion, although recent advances in theory and experiment suggest failure is history dependent. The theory concerns components that operate in nominal line contact and have surface roughness with a lay parallel to sliding. However, the predictions are independent of sliding speed, the observations consistent with micro-ehl could be attributable to other temperature dependent effects, some variations in contact frequency are difficult to interpret, and running-in remains unpredictable. In the present work, the capabilities of an existing twin disc machine were significantly enhanced. It was found that temperature dependent effects other than micro-ehl were relatively minor. Changes in contact frequency were associated with transitions between ehl, mixed lubrication and micro-ehl, but were dependent on the presence of a distinguishable "mainscale" wavelength in the surface roughness. A prior surface smoothing process of running-in delayed the onset of scuffing, and surface blackening accompanied running-in in mixed lubrication. Surfaces machined to have a dominant mainscale wavelength without secondary features showed little change. To investigate the effects of surface speeds on scuffing, operating conditions were restricted to test histories that induced mixed lubrication. Two regimes of behaviour were identified within a sliding speed/rolling speed domain. At higher sliding speeds, discs scuffed close to transition from ehl to mixed lubrication. At lower speeds, they ran-in and did not scuff until relatively severe, micro-ehl conditions were imposed. A theoretical heat transfer model was matched to the boundary between the two regimes. Refinement of the model suggested the criterion for scuffing in mixed lubrication to be that the load and speeds allow asperities sufficient time within the conjunction to experience a temperature rise large enough to cause local welding.