Each vehicle has fluid and most have serviceable filters that require changing/servicing at either 30,000 or 50,000-mile intervals. As fluids age, they break down and stop lubricating as needed, so this service helps prevent any expensive breakdowns of the internal transmission components.
Differential Fluid Services. The rear differential, as well as the front (on 4wd and AWD vehicles), require scheduled fluid services to replace aged fluids to assure proper lubrication of all internal gearing, normally every 30,000 miles on most vehicles.