Hexagonal boron nitride (h-BN) nanosheets are widely used as key additives to enhance the performance of lubricants under harsh working conditions because of their excellent high-temperature resistance, good chemical stability, and layered structure similar to that of graphene. Recently, diurea modification was shown to endow h-BN nanosheets with unique ultrasonic-induced thickening properties, enabling their use in the formulation of ultrasonic-responsive nanogel lubricants. However, the mechanism by which diureas regulate the thickening properties and lubrication performance of h-BN nanosheets is still not well understood. Therefore, in this work, h-BN nanosheets were modified with three diureas with straight-chain structures, and the effects of chain length on the ultrasonic-induced thickening capabilities and lubricating properties of polyalphaolefin 8 (PAO8) were investigated. The thickening capabilities of the modified h-BN nanosheets were found to depend mainly on the interaction between diureas and PAO8. Tribological tests revealed that the nanogel formed by the modified h-BN nanosheets under sonication exhibited better lubrication performance than PAO8 under startup, varying speeds, and heavy load conditions. The high fluidity and high h-BN content were found to be essential for attaining the optimized lubrication performance of h-BN nanogels.