How Often Should You Change Your Car’s Fluids?

Oil, coolant, brake, and transmission fluid are key parts of regular maintenance.

Woman mechanic changes car fluids under the hoodShutterstock

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Staying on top of fluid maintenance is one of the simplest ways to protect your vehicle and extend its life. Paying attention to the service intervals for each of your automobile's major fluids can be inexpensive insurance against the wear and tear caused by regular commuting. For those who drive their vehicles under more extreme conditions, such as plowing snow, towing a trailer, or visiting the racetrack, fluid maintenance becomes even more important.

Here's a look at the three major vehicle fluids and how often they should be changed.

Engine Oil: 3,000 to 15,000 Miles

For internal combustion vehicles, changing the engine oil is considered one of the most important fluid services. A vehicle's engine oil is designed to protect the moving parts inside the motor from friction, and its ability to lubricate breaks down over time and mileage.

For the exact oil change interval for your specific vehicle, the owner's manual will always have the precise service window, as well as whether you should be using a synthetic or a conventional motor oil. In general, the window ranges from 3,000 to 15,000 miles. That’s a broad range that varies greatly by automaker, and it’s not necessarily a hard guideline.

Oil-change intervals can also be affected by driving conditions. An engine that often sees duty towing or hauling, or which hits the racetrack on a regular basis, will see its oil degrade more quickly. The same is true for a vehicle that is driven in dusty, off-road conditions, as that grit can make its way past filters and into the engine. These conditions fall under the severe duty section of the oil change window in an owner's manual.

Transmission Fluid: 30,000 to 60,000 Miles

Manual transmission fluid is similar to engine oil, in that it lubricates (and also cools) the gears inside the transmission. Most manual transmission fluid replacements should occur between 30,000 and 60,000 miles.

Automatic transmission fluid is different. This pressurized hydraulic fluid actuates gear shifts, while also cooling and lubricating. Automatic transmission designs are more complex than manual transmissions, and their window for fluid replacement is considerably wider: 25,000 miles to 150,000 miles, depending on the model.

As with any fluid, excessive heat or heavy-duty use can accelerate the need to change it in either type of transmission. Ignoring your vehicle's suggested fluid swap window can lead to poor transmission performance or even failure.

Radiator Coolant: 30,000 Miles or Two Years

A vehicle's cooling system relies on a mix of water and coolant to protect the engine from overheating. Over time, the heat generated from driving can cause that coolant to deteriorate, lowering its boiling point and increasing the risk of your engine's temperature rising to an unsafe level. Engine coolant additives help fight corrosion inside a motor, and they also wear out over time.

Just as with engine oil, a vehicle's owner's manual will include a mileage window for coolant replacement. You can also test coolant with a simple tool to determine whether it's still providing enough protection. Even coolant that looks and tests as good should generally be replaced every 30,000 miles or every two years, whichever comes first.

Brake Fluid: 30,000 Miles or Two Years

Modern cars — even those with electronically-assisted brakes — use hydraulic fluid to activate the pads that grab wheel discs (or cylinders in cars with drum brakes). This fluid is hydroscopic, which means it can become contaminated with water. That in turn leads to a risk of brake lines rusting and eventually developing leaks.

An engine-oil leak might be a nuisance; a brake-fluid leak could make it very difficult to stop.

Automakers usually schedule two- or three-year intervals when it comes to flushing out old brake fluid. As with the other fluids on this list, however, more braking system use can mean a quicker breakdown of the fluid itself. If it has turned from a light yellow to a dark brown, a mechanic may suggest an earlier service.

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Benjamin Hunting
Benjamin Hunting is a writer and podcast host who contributes to a number of newspapers, automotive magazines, and online publications. More than a decade into his career, he enjoys keeping the shiny side up during track days and always has one too many classic vehicle projects partially disassembled in his garage at any given time. Remember, if it's not leaking, it's probably empty.