Reversely identical transmissions for a tiltrotor aircraft include reconfigurable gearbox lubrication systems to accommodate operation in either direction of propeller rotation, using common parts. In a typical installation, at least two separate winged tiltrotor transmissions operate with opposing propeller rotations. Common parts of the two transmissions include cast housings, rotor shafts, pinion gears, lubrication sump tanks, lubrication pumps, and lubrication filter manifolds including filters having reversible orientations. Either transmission configuration can be utilized on either the left or right side of a tiltrotor aircraft by swapping the lubrication sump tank and lubrication filter manifold locations on the gearbox. In addition, a plug is used to manually reconfigure an opposite lubrication oil path to accommodate first and second rotational directions of the lubrication pump.