The differential of a vehicle is the set of gears that transfer the power of the transmission to the tires. The purpose of the differential fluid is to cool and lubricate your differential. Without the oil your differential would overheat and burn itself out. This oil is different from your engine oil and transmission fluid because it is thicker. We usually inspect the fluid level whenever we do an inspection or service a vehicle. For the most part, a fluid level check twice a year should be good.