Can Transmission Fluid Be Used as Power Steering Fluid?
4 Answers
Using transmission fluid as power steering fluid, transmission fluid primarily serves a lubricating function. Hazards of not changing transmission fluid: 1. Damage to the gearshift control mechanism of the transmission; 2. Abnormal wear of gears and clutch plates inside the transmission; 3. Affects the normal operation of the transmission, leading to damage of the transmission valve body. Transmission fluid is different from engine oil. Although both fluids serve a lubricating purpose, their effects are distinct and should not be confused. Differences: Engine oil is the lubricating oil for the engine system, possessing certain properties such as oxidation resistance, wear resistance, and corrosion prevention. Transmission fluid, on the other hand, is mainly used to keep the gear system clean and extend the service life of internal transmission components. Due to the different structures and operating environments of transmissions and engines, transmission fluid cannot be used as engine oil, nor can engine oil be used as transmission fluid.
I've specialized in automotive fluids, and transmission fluid and power steering fluid are definitely not interchangeable. The steering system requires extremely precise pressure control, and transmission fluid simply can't meet the high anti-foaming performance requirements. I've seen car owners mix them during repairs before, resulting in loud pump noises and extremely heavy steering. The key issue is the vastly different additive formulations - the phosphorus and sulfur compounds in transmission fluid will corrode the rubber seals in the steering gear, inevitably causing leaks over time. If you're really in an emergency without power steering fluid, it's better to temporarily use high-grade hydraulic fluid as a substitute, but you must thoroughly flush the entire system afterward and replace it with the proper specialized fluid.
As a veteran driver with twenty years of experience, I strongly advise against doing this. I remember once driving an old Passat on a long trip when I couldn't find power steering fluid at the service station, so I tried mixing in some transmission fluid as a temporary solution. After just 300 kilometers, the steering wheel became noticeably heavier, and I heard a whining noise. I rushed to a repair shop to have it flushed, and the mechanic found that the seals inside the steering gear had swollen and deformed. The technician was right—the steering system is highly precise. The specialized fluid not only lubricates the rack and pinion but also protects the vanes inside the power steering pump. Transmission fluid's viscosity isn't compatible, and in summer, it can cause vapor lock, leading to power steering failure, which is extremely dangerous.
The power steering fluid must meet ATF or PSF specifications, while transmission oil is mostly ATF or MTF grade. I usually judge by fluid parameters: steering fluid requires higher shear stability, otherwise the vane pump rotating at 2000 rpm would quickly emulsify and deteriorate; it also needs lower cold-start viscosity to ensure easy steering even at -40°C. Most crucially, the friction coefficient—the steering system demands a static-to-dynamic friction ratio close to 1, otherwise the steering wheel feels sticky. Transmission oil tuning prioritizes synchronizer shift smoothness, with completely mismatched parameters. Using the wrong oil will make the steering gear whine within three months, and hydraulic lines may even burst.
