yea, it's an old "falacy" that you don't need to change diff and stick trans fluid. But besides lubricating, the oils in these also suspend the metallic particles that eventually wear. So it's a good idea to change them. Some race teams will strain the fluid through a pro paint strainer and reuse the "syns" - either way getting the metal paticles out will add to the longevity. Also when the fluids age then tend to foam & thin out.