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We used the fiber wobbling method (FWM), which can measure a small shear force of 0.1 nN at high shear rate, to measure the viscosity of perfluoropolyether (PFPE) lubricants confined in nanometer-scale gaps. In a previous paper, we demonstrated that the viscosity of PFPE lubricant Z-DOL increases due to confinement. To clarify the mechanism of the viscosity increase, we tested PFPE lubricants with different types of end groups, i.e., Z03, Z-DOL, and A20H. Our experimental results indicate that there are two different types of mechanisms: reduced mobility of mobile molecules and interaction of immobile molecules. The former causes the viscosity increase at wider gaps of 5 nm to 40 nm in Z03 and Z-DOL, and the latter causes the increase at gaps of less than 5 nm in Z-DOL and A20H.