Abstract: Using the ball-on-disc optical lubricating film measurement device, the evolution of oil reservoir patterns near pure rolling contact was observed. It was found that three patterns of oil reservoirs, i.e., closed, half-opened, and fully separated were formed under low speed, moderate speed, and elevated speed, respectively. For the closed pattern, the dimensionless cavitation length almost proportionally increased with the increase in the entrainment speed. There existed a critical entrainment speed beyond which the rear reservoir was split by a cavitation bubble. It was the parameter of surface tension at the boundary of the cavitation bubble, that resulted in the deviation of predicted cavitation length from that of measured one. The entrainment effect and the pressure difference cause the deformation of the oil reservoir shape. The lubricant feed-loss balance was determined by the oil charging amount and entrainment speed. Consequently, the trends of the width of the side oil reservoir with entrainment speed were different.