Answer: Differential fluid is probably the most overlooked fluid in your vehicle. Over time, the heat and friction produced by the moving differential parts will break down the differential fluid. Small particles of the gears and shafts can also break off and mix with the fluid. Overused differential fluid can lead to poor differential performance/noise, excessive wear, the breakdown of the gears/shafts in your differential, and eventually, complete differential failure. Complete differential failure would require replacement of your differential system, which can be quite costly.