Modern vehicles usually come with two types of automotive transmissions: automatic or manual. 

Manual transmissions put gear changes in the hands of the driver, allowing them to switch ratios manually, typically via stick shift. Automatic transmissions take care of gear changes themselves, letting the driver simply drive and fully focus on the road. 

Automatic and manual transmissions work differently, and as a result, require unique types of transmission fluid. Manual transmission fluid is similar to gear oil, lubricating pieces like bearings and gears. 

But automatic transmission fluid (ATF) plays a more significant role in transmitting engine power to the road. In an automatic transmission, a hydraulic torque converter takes the torque from the engine and changes it into usable power for forward motion. 

ATF works inside the torque converter, allowing this conversion to occur and turning the engine's power into energy to move the wheels. Since automatic transmission fluid is so essential to transferring power, it typically degrades faster than the fluid in manual transmissions, requiring more frequent changes. 

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As the miles add up, transmission fluids may pick up metal shavings and gunk from gears and other moving parts, lessening performance. That can cause the transmission oil to lose some of its cooling capacity and lubrication properties. 

When you go through the gears, transmission problems like slow changes between ratios or a slipping sensation may indicate the need for new transmission fluid. Poor gear change quality can also signify deeper mechanical issues. 

Though changing transmission fluid can't fix mechanical problems, it does make for smoother shifts and can extend the life of your transmission. 

And even when manufacturers recommend fluid change intervals of 150,000 miles under normal conditions, it may be a good idea to change it more frequently. Some mechanics believe 150,000 to be too long of a service interval and advise fresh fluid on a more regular basis.

The cost for servicing a transmission varies, depending on the vehicle's make and model, whether you need a drain or a flush, and if your car requires a new transmission filter.

While a repair shop might advertise a fluid change for $100, some luxury cars like a Mercedes or BMW run much higher, costing up to $400 or more. The price of servicing a transmission also depends on whether you receive a flush or have the old fluid drained and refilled. 

With a transmission fluid flush, a service center pumps in new fluid, which moves out the old fluid, along with any contaminants. Draining and replacing the old fluid may not be as thorough as a flush — possibly only removing and replacing half of it — but that could be all that your manufacturer requires.

Just like changing the engine oil per manufacturer's recommendations extends the life of your motor, the same goes for changing transmission fluids.

Motor oil degrades over time with each turn of the crankshaft, and so does transmission oil as gears change, and the torque converter handles heavy loads. But when is it time to change transmission fluid?

Maintenance schedules vary among manufacturers, but they all have one thing in common: heavy use requires more frequent flushing of transmission fluids.

For example, here are some guidelines from major manufacturers on when to change transmission fluids:

• Toyota advises changing the transmission fluid every 60,000 miles under "severe" conditions. But if you're simply running errands or taking trips, Toyota doesn't require flushing a transmission.
• General Motors (Buick, Cadillac, Chevrolet, and GMC) recommends changing the transmission fluid every 45,000 miles if you experience "severe" conditions, like towing or other heavy use. However, GM doesn't mandate changing the transmission fluid when driving under normal conditions. 
• Honda uses continuously variable transmissions (CVT) on many vehicles, and a Maintenance Minder follows miles driven and time and sounds an alert if you need to change the transmission fluid. But for drivers who live in the mountains or regularly tow a boat or trailer, Honda suggests changing transmission fluid every 25,000 miles.
• BMW recommends changing transmission fluid every 60,000 miles.
• Ford encourages changing the transmission fluid at 150,000 miles if you drive under normal conditions.
• Mercedes-Benz recommends changing transmission fluid at 40,000 miles.
  

With automakers advising a range of mileage intervals for servicing a transmission, you may wonder when to replace your transmission oil. The first place to look is your owner's manual. There, you'll find specific recommendations for your vehicle and when to change transmission fluids to ensure its longevity.

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