Let’s continue the topic of automotive lubricants and focus on transmissions—specifically, on the nuances of replacing the working fluid in automatic transmissions (AT) and continuously variable transmissions (CVT).
1. How Often Should Transmission Fluid Be Changed?
Is fluid replacement even necessary? According to the owner's manuals of many modern vehicles, transmission fluid is "lifetime fill" and does not require replacement. While technically true, this “lifetime” may turn out to be surprisingly short—for the transmission, that is.
Like any fluid in your vehicle, transmission fluid degrades over time due to oxidation, contamination from wear particles, and thermal breakdown. This deterioration accelerates wear on clutch packs, causes solenoid overheating, and leads to harsh shifts or shuddering. Ultimately, it significantly shortens the lifespan of the transmission.
Recommended service intervals:
- Automatic Transmissions (AT): every 40,000–60,000 km
- CVTs: every 30,000–50,000 km
If your vehicle is used for towing, frequent hill starts, or driving in stop-and-go traffic, aim for the lower end of those ranges.
2. Full Flush or Drain-and-Fill?
Most service shops will recommend a machine-assisted fluid exchange, which replaces nearly all of the old fluid. This is a good option if the transmission isn’t too old and has received at least some maintenance over its life.
However, for high-mileage vehicles with original factory-fill fluid, a high-pressure flush can do more harm than good. It may dislodge accumulated sludge and debris, clogging the valve body. In such cases, it’s safer to perform a series of drain-and-fill services: drain a portion of the old fluid, refill with fresh fluid, drive the vehicle briefly, and repeat the process.
3. How to Choose the Right Transmission Fluid
Fluids differ significantly between transmission types.
- ATFs (automatic transmission fluids) contain anti-wear and friction-modifying additives tailored for planetary gearsets.
- CVTFs (continuously variable transmission fluids) focus on anti-foaming agents and friction modifiers to prevent belt slippage and overheating.
Even within the same transmission type, fluids vary by viscosity and additive packages.
Older transmissions often used thicker fluids for better lubrication at low RPMs—but at the cost of efficiency. Newer designs favor low-viscosity fluids to reduce parasitic losses and improve fuel economy.
Always use a fluid that meets the OEM’s specific approval for your transmission. For more details, see the breakdown of ATF and CVTF fluids from Masuma.
4. Do You Need to Replace the Transmission Filter?
In most automatics, the transmission filter is located inside the case and only accessible by removing the transmission pan. In some models, disassembly of the transmission is required. Despite the hassle, filter replacement is essential—a clogged filter restricts fluid flow, leading to overheating and erratic shifting.
Some vehicles come without a transmission filter, but the housing may have a dedicated port for installing one (often sealed with a factory plug). If you operate in severe conditions, consider retrofitting an appropriate inline filter to extend transmission life.
Masuma offers nearly 400 different types of transmission filters.
5. How to Properly Check Transmission Fluid Level
Maintaining the correct fluid level is critical—even minor deviations can affect transmission performance. Each transmission type has its own procedure, but the fluid level must always be checked with the engine running, so the fluid is circulating.
- Transmissions with a dipstick:
Check fluid after the transmission reaches operating temperature (typically after 15–20 minutes of driving). Park on a level surface, cycle through all gear positions, then leave the selector in “P” or “N”. Remove the dipstick and verify the fluid level against the “HOT” markings. - Transmissions without a dipstick:
Use the fill/check plug on the transmission housing. Warm the fluid to the temperature specified in the service manual (usually measured via scan tool or diagnostic software). Remove the plug: intense fluid flow indicates overfill, while no fluid means the level is too low. A light trickle generally means the level is correct.