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When and How You Should Maintain These 5 Vehicle Fluids

Updated: Apr 28, 2021

Need a New Year’s Resolution? How about being proactive with your vehicle maintenance in 2015? If you don’t remember the last time you opened the owner’s manual to check the service schedule, this could be a good resolution after all. The least you can do to make sure your car runs well is to maintain its fluids and lubricants. And as specialists in automotive maintenance, our pros at Hillmuth are happy to give you a few pointers.

Engine Oil

It’s close to impossible to own a car without being aware of the necessity of oil changes. Oil is what keeps your engine lubricated and reduces the friction and wear of the engine’s components. Subject to high heat, oil eventually loses its viscosity and may clog up your engine if it gets too burnt and dirty. Both oil and the oil filter need to be replaced at certain intervals—the frequency will depend on your car’s age, your driving habits and the type of oil it uses.

If your car is under five years old, uses synthetic oil and you don’t drive it in extreme conditions, you could get away with oil changes every 7,000 to 15,000 miles. The old 3,000-mile rule is no longer applicable to modern vehicles, because both the engines and the oil have evolved to be more durable. Refer to your owner’s manual for the manufacturer’s recommendations since every car is different. Some newer cars come with a dashboard oil health indicator, while in older cars you can always check the oil level and color with a dipstick.

Transmission Fluid

Transmission fluid lubricates the transmission assembly and helps you shift gears. Whether you have a manual or an automatic transmission, it runs thanks to the transmission fluid. The type of fluid that goes into your car will depend on the type of transmission. It’s crucial that the right type is used, which is why you should trust transmission fluid flushing to an experienced Maryland mechanic like our pros here at Hillmuth Certified Automotive.

Once again, check with your manual for the recommended intervals. For manual transmissions it could be between 30,000 and 60,000 miles. Some automatic transmission fluids are “filled for life” and technically don’t require replacement. However, keep in mind that the manufacturer’s definition of the vehicle’s lifespan may differ from what you want to get out of your car. If you don’t want to have transmission problems around 140K miles, check and flush your automatic transmission fluid about every 30K miles. Transmission fluid is the second dipstick under your hood—if the liquid is dark cherry-red instead of pink, it’s probably time to replace.

Brake Fluid

Like most of the fluids in your vehicle, brake fluid gets hot and deteriorates, which can lead to reduced brake performance. Many manufacturers suggest replacing brake fluid every 20,000 – 30,000 miles or every 2-3 years. In some manuals you won’t find anything about the brake fluid, which means you can assume it was filled for the life of the vehicle. Talk to your mechanic if you are not sure whether you should change your brake fluid at all and when. If no guidance is offered by the manufacturer, it’s still a good idea to replace brake fluid with your regular brake service, such as brake pad replacement.

Differential Oil

Your car’s differential is a “gearbox” that helps control the speed of wheel rotation when your vehicle is turning. Because the outer wheels cover a longer distance during a turn, they need to rotate faster than the inner wheels to ensure smooth turning, and that’s what a differential does. All cars have some sort of differential. In front-wheel-drive cars it’s typically referred to as a “transaxle,” is located underneath your car in the front portion and shares the same liquid as your transmission.

In rear-wheel-drive vehicles the differential is in the rear and uses its own lubricant—the differential oil. There’s no shame if you’ve never heard of it, as it’s not something that requires frequent servicing. If you change your cars every few years, you might never get to the point when differential oil replacement is necessary. However, if your vehicle has passed the 30K mark, you might want to check your manual. Your differential oil replacement intervals will depend on your vehicle and its primary use. For example, cars that tend to get their undersides wet frequently by pulling boats out of lakes may need more frequent differential fluid service.


Coolant (radiator fluid) is the 50/50 antifreeze and water mixture that helps cool the engine and prevents it from overheating. It should be checked and refilled as needed and replaced every several years. The frequency will depend on your particular driving habits. Over time, the coolant mixture may become acidic and lose its effectiveness—ask your Maryland mechanic to perform a litmus test to determine the condition of the liquid. Don’t confuse the coolant with refrigerant, which is freon used to generate cool air when you turn the AC on. Refrigerant shouldn’t have to be refilled unless there is a leak that causes levels to go down.

For your convenience, Hillmuth Certified Automotive has developed milestone service packages. When your car hits 3K, 7K, 15K or 30K, you can bring it in and we’ll take care of all required maintenance, including fluids. For older cars, we can helps you assess the need for specific fluid service based on your car’s condition. Feel free to stop by one of our Maryland auto repair shops in Clarksville, Columbia, Gaithersburg or Glenwood for vehicle fluid assessment.

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