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A variety of factors can affect the frequency with which you should change your auto transmission fluid (ATF). Different types of vehicles may have varying service intervals, and your owner's manual may specify the correct mileage for your car. Many vehicles will need their auto transmission fluid changed about every 25,000 miles (40,000 km), though some will specify a longer interval. Driving conditions may also require more frequent changing, and checking the fluid regularly will usually show if there has been substantial fluid degradation. This sort of regular inspection can also reveal if the fluid is low, which typically indicates a leak.
Auto transmission fluid typically gets very hot during operation, which can leads to a breakdown in its composition and interfere with its ability to lubricate. Various internal transmission components may also wear out over time, releasing contaminants into the fluid. Brand new ATF is typically a bright, cherry red color, while dirty fluid can appear grey or black and may smell burnt. Wiping dirty fluid on a clean cloth will often reveal metal particulates as well. If any of these characteristics appear in your auto transmission fluid, it may be time to change it regardless of mileage.
An important part of a transmission flush is replacing the filter, if one is equipped. A few vehicles have external, spin on units similar to many oil filters, though it is more common for transmission filters to be located internally. In this case, the transmission pan must be removed in order to replace the filter. If the pan gasket has developed a leak, it may be a good idea to have your fluid changed and the filter replaced at the same time the gasket is repaired.
There is typically no harm associated with changing ATF more often than the suggested mileage. The operation can be quite costly, but changing it before the fluid becomes contaminated may help extend the life of the transmission. When a flushing service is performed more often than the suggested interval, it may not be necessary to replace the filter each time.
Manual transmission fluid is typically a little different from auto transmission fluid. A few manual transmissions do use ATF, but most use heaver motor oil or hypoid gear oil. Typical manual transmissions do not get as hot as automatic transmissions, but contamination from wearing parts can still be a concern. Many manufacturers suggest a service interval of 30,000 miles (48,000 km) for manual transmissions, though certain heavy usage may require more frequent servicing.