in the manual subaru AWD system, there are 3 differentials. Only the center one needs to be a LSD. In all current US market Subarus, the center diff is a viscous coupling unit. this means that as one drive shaft spins faster then the other (indicating slip) a fluid in the diff gets heated up and becomes thicker, creating more resistance against the spinning driveshaft and thus encouraging torque to go to the other driveshaft (from the wheels that slip to the wheels that grip). In some cars, such as the MY98-99, the front and rear diffs are open. this won't cause any type of accessive wear, it will just produce less traction because you then have to have both fronts or both rears maintain traction if one end looses it. with LSDs at the ends, then you can still pull yourself out with only one wheel still having traction. in the MY00-01 RS and the US-spec WRX, the fronts are open and the rear is another Viscous Coupling LSD. In cool places where they get things like the STi and 22b, they have all kinds of crazy mechanical diffs which are far better, and in some cases for the center diff, you can manually adjust the torque split. very trick shit. and then there are WRC cars which use electronically controlled diffs to constantly vary the torque distribution to maintain maximum traction.