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It’s no exaggeration to say that automatic transmissions run on oil, more properly called Automatic Transmission Fluid, or ATF. Because of its dependence on ATF, low fluid level can have a disastrous effect on transmission operation — and even transmission life.
Unfortunately 2.8 liters of the fluid stays in the transmission when you drain it so it takes a while to flush that stuff out and get the transmission clean. My mistake was not waiting 1000 miles in between changes to let the fluid wash the transmission.