Your transmission fluid is a critical lubricant for the many different components within the system. Transmission fluid helps to minimize friction within these parts but wears down over time. Change intervals vary depending on the vehicle and whether you are running an automatic or manual transmission. In general, newer automatic transmission vehicles need to have a transmission flush between 60,000 and 100,000 miles but some vehicles are actually made to never have a flush. Manual transmissions tend to need to have their fluids changed more often, every 15,000 miles or so for heavy-duty vehicles and every 30,000 to 60,000 miles for general duty vehicles.