Most mechanics recommend transmission fluid replacement every 60,000 miles for Mazda 323 cars with an automatic transmission. Manual-transmission cars may require fluid replacement every 60,000 miles or even more frequently under heavy-duty use.
Manual transmissions generally require more frequent fluid replacement than automatic transmissions, largely due to the buildup of contaminants from wear on the gears and synchronizers in the transmission. These contaminants take the form of small metal particles that can cause excessive wear on the transmission if the fluid is not changed regularly. While 60,000 miles is a general guideline, owners of older cars and Mazda 323s used for city driving should consider changing manual transmission fluid every 30,000 or even 15,000 miles.