<p>The literature broadly examines lubricated joints in multibody systems, treating viscosity as one influencing factor but lacks dedicated focus on its specific impact on finite (with side leakage) joint performance, often relying on simplified models that cause discrepancies. This study examines the effect of lubricant viscosity on the steady-state and transient behavior of a Finite-Length joints with clearance and side leakage. It also investigates the impact of viscosity on the overall performance of multibody systems incorporating such joints. An analytical framework, derived from an exact analytical solution of the Reynolds equation, is employed to capture nonlinear, viscosity-dependent phenomena governing hydrodynamic force generation and eccentricity in lubricated joints. Results show that higher-viscosity lubricants enhance film thickness, which reduces wear, but increase friction and thermal issues, while lower-viscosity lubricants minimize friction but risk film integrity and load capacity. The study delineates the influence of lubricant viscosity on hydrodynamic film integrity and transient torque peaks in multibody systems, highlighting its role in modulating system stability and performance. Numerical simulations reveal that eccentricity and torque peaks in multibody systems vary with viscosity, necessitating optimized lubricant selection to balance friction, thermal stability, and load-bearing capacity. These insights provide a robust foundation for the design and optimization of mechanical systems, enhancing durability, efficiency, and reliability in high-speed, high-load applications.</p>