 Transmission fluid, for example, prevents wear of gears, bearings and bushings, protects against corrosion, and acts as a heat transfer fluid. You may not think about this every time you get behind the wheel, but as soon as it's not doing its job, you'll notice. Misapplication of transmission fluid can mean reduced fuel economy and abrupt or hard shifts if the static friction is too high. It can also cause sluggish operation at low temperatures if the viscosity is too high and leaking through seals if viscosity is too low.