Is the Transmission Fluid the Same for 5AT and 6AT?

[AlexDella]

5AT and 6AT transmission fluids are not the same. Additional details are as follows: 1. Transmission Classification: Transmission types include manual transmission (MT), semi-automatic transmission (AMT), automatic transmission (AT), continuously variable transmission (CVT), and dual-clutch transmission (DCT). These different types of transmissions use different transmission fluids. MT and AMT transmissions generally use the same transmission fluid because their working principles and transmission structures are fundamentally similar, hence both use MT transmission fluid. AT, CVT, and DCT transmissions typically require their own specialized transmission fluids. AT transmissions mainly use ATF (Automatic Transmission Fluid) specific oil. Currently, AT transmissions include 4AT, 5AT, 6AT, 8AT, and 9AT models, and the types of transmission fluids they use also differ. Generally, the higher the number of gears, the higher the requirements for the ATF oil. CVT transmission fluid is essentially similar to ATF oil, but CVTs also use specialized fluid because their transmission structure relies on a steel belt, requiring specific properties in terms of fluid flow rate and expansion coefficient. Therefore, CVT transmission fluid cannot be mixed with ATF oil. DCT transmissions have a fundamentally different structure, consisting of a gear-based mechanical transmission system, and can be thought of as a combination of two MT transmissions. Thus, DCT transmissions use two types of fluids: gear oil and hydraulic oil, both of which are specialized and cannot be mixed with other transmission fluids. 2. Function of Transmission Fluid: Taking the common AT transmission as an example, the primary purpose of transmission fluid is to lubricate the various components of the transmission. Since a car is a consumable product and the transmission is responsible for power output, the wear and tear on transmission components can be significant. Additionally, gear shifting in a transmission is achieved through the coordinated action of the transmission control mechanism, which relies on the hydraulic properties of the transmission fluid. Therefore, transmission fluid also serves a hydraulic transmission function. Furthermore, transmission fluid helps with heat dissipation, enhances component tensile strength, and reduces mechanical impact to some extent.

[StMason]

I've researched this for a friend before. The 5AT and 6AT transmissions most likely use different fluids, even though there's just one gear difference. AT transmission fluid viscosity requirements are extremely precise - the more complex 6AT structure often requires higher fluidity oil to properly lubricate the additional planetary gear sets. An experienced mechanic once told me that while older Japanese 5ATs use DW-1 fluid, newer 6ATs require DW-2. Mixing them could cause anything from rough shifting to burnt clutch plates. The safest approach is to either follow the maintenance manual or provide your VIN to parts suppliers to verify the correct specification. It's far more cost-effective than trying to save a few bucks and ending up with transmission repairs.

[Holden]

As a car owner who frequents repair shops, let me tell you: the chance of them being different exceeds 80%. My classic Focus 5AT used MERCON V standard, but when I switched to the Edge 6AT, it required MERCON LV – the parameters of these two fluids differ significantly. The key lies in shear resistance – the 6AT has an additional set of friction plates, so the fluid film needs to withstand more pressure. Last time, the repair shop guy gave me an on-the-spot lesson: using the wrong fluid can cause the valve body to fail prematurely, and the repair bill could buy two cases of top-tier transmission fluid. Now I always purchase according to OEM part numbers – for instance, Honda's HCF-2 and ATF-DW1 are clearly not interchangeable.

[IsabelRose]

With a decade of experience in the repair shop: they are basically not interchangeable. The additional planetary gear set in 6AT requires stronger oil cooling, so the viscosity index is typically about 10% lower than 5AT oil. A few months ago, we encountered a Camry owner who mixed them, causing oil foaming and upshift slippage. Another key point: modern 6ATs like those in the Sonata use SP-IV certified oil, which is completely incompatible with the SP-III additives in older 5AT oils. Simply put, checking the sticker number on the side of the transmission or referring to the oil specification table on page 33 of the maintenance manual is the most reliable method.